THE Entrochi comprehend a class of fossils, the recent characters of which are not well known. they have obtained various names, as Screw-stones; Fairy-beads, of the vulgar in England; Witch-beads, of the vulgar in Scotland; by workmen in Kilbride they are more properly called Limestone-beads. They are frequently called St. Cuthbert‘s beads, from a vulgar opinion that they were made by the holy man; or because they were used in the Rosaries, worn by the devotees of that Saint.1 It is not in England only that they have obtained a name from a Romish ecclesiastic: on the continent they have been known by the name of Nummuli Sancti Bonifacii. Linnæus considering this fossil as the remains of an animal, which he ranks under the Zoophyta, calls it Isis Entrocha stirpe testacea tereti, articulis orbiculatis perforatis, ramis verticillatis dichotomis.2 The name Entrochi was, more than two centuries ago, given them by Geo. Agricola. The propriety of the name appears from their construction; for they consist of a great number of beads, or whirls connected with one another by means of futures, or minute striæ. These beads, when found separately, are called Trochitæ. They are composed of calcareous rhomboidal spar, of a lamellated texture, but the lamellæ diverge from the center of the entrochi in an oblique direction. By Hill (Hist. of Fossils, p. 653.) it is called an obliquely arranged, tabulated spar. It readily dissolves in acids, leaving a little clay, or selenite. At first they were supposed to be Lapides sui generis, originally produced in the earth. This theory was too gross to be long retained by careful observers of nature. It was soon believed that they were productions of the ancient ocean: but whether they belonged to animals or vegetables, was uncertain. They are now generally thought to be Vertebra of the Encrinus, Medusa, or some such animal. Their true history, however, is far from being well known.
THE entrochi are commonly of a whitish gray colour; but in general they partake of the same tinge with the substance in which they are inclosed. From several specimens it would appear that their constituent parts were a Medulla, a Cortex, and what may be called the main body, that lies between, and resembles the ligneous part of a plant. Examples of this are given in fig. 8, 10. pl. XVII. a, a is part of the cortex adhering to the main body. The specimens from which I took the figures had, probably, been partly decomposed, prior to the time when they became fossil. These component parts seem originally to have possessed different degrees of solidity, by which some of them were able to resist, for a longer time than others, the action of the corroding matter, to which they were exposed.
THE proportion which the medulla bears to the diameter, is not alike in every specimen: nor are the medullæ uniformly of the same shape. Some of them are oblong, as in fig. 1. In this variety they change their direction in the same bead, for if, on the one side, the medulla lies in the direction as at a, it will take, on the other, the direction, as at b. If the medulla is pervious, which it does not appear to be, the contortion must be exceedingly quick, since the space in which it is made, does not sometimes exceed the eighth part of an inch. In some few specimens the medulla is triangular, or quadrangular, as in fig. 2, 3. and sometimes hexangular, but this is extremely rare. In some it is pentangular, fig. 4. but in far the greatest number of specimens, it is round, as in fig. 5. Brachia adhering to specimens of the pentagonal kind, have sometimes an oblong, or triagonal medulla. This part pf their structure, therefore, is an improper mark on which to fix their classification.
NEXT to the medulla is the main body; draughts of which are given fig. 6, 7, 9. Its proportionable thickness is different, in different specimens: in general, however, it is about ¼ of the diameter. At the divisions between the beads, it projects farther than in any other part, as at a, fig. 6. This prominency enters a corresponding cavity in the cortex. It frequently has two thin plates, one on each side, running the whole length, an example of which is given at a, fig. 7.
THE cortex is sometimes found detached from the other parts, as in fig. 13. The concave surface, which, in the figure, is exposed to view, and is full of ridges and furrows, is wholly covered with striæ, or sutures, answerable to corresponding ones in the main body.
THE striæ, with which this class of fossils is always ornamented, are mostly bifurcated, and very distinct. They are seldom visible in fractures, newly made. It is hardly possible to dislocate the beads from one another, without violence to their structure: and the fractures are seldom horizontal, but extremely irregular. But as vast numbers of the beads are found separately, without any injury in the most minute striæ, and as it is impossible to make such a separation, in their present state, it follows that these dislocations were made, prior to the time when the entrochi became fossil. They must also have been made with very great ease, since the finest and most tender of the beads were in no danger of being broken, or the extremely minute sutures injured in any respect. Many of these dislocations appear to have been made by a pressure acting equally upon the entrochi, at the same time, as fig. 2, 3. pl. XVIII. This curious fossil seems to have been frangible, in a recent state; and capable to receive compressions and fractures, of which living shells are susceptible, fig. 6.
SPECIMENS of entrochi are of all intermediate sizes, from the thickness of a small pin, to an inch in diameter: and some specimens are five inches long. They are all in fragments, lying confusedly in every direction, and imbedded in various substances, as lime-stone, free-stone, till and clay. The peculiarities of the whirls or beads are considerable. In the specimen, fig. 15. pl. XVII. they are equally thick, without any convexity or depression in the middle. In fig. 20. they are likewise equally thick, but rise to a sharp ridge in the middle: in specimens of this kind the striæ are remarkably fine. In the specimen, fig. 11. they are regularly thicker and thinner, and more or less prominent in proportion to their thickness. the round and pentagonal medullæ belong indiscriminately to this variety. Sometimes the whirls are concave, fig. 14. at other times convex, fig. 12. In these varieties the oblong medulla is very frequent. The species, fig. 5. pl. XVIII. is pentangular, with a round medulla. The surface of some varieties, as fig. 19, 22. pl. XVII. is rough, owing to small papillæ. In some specimens the whirls are extremely thin, fig. 21. The specimen from which the draught was taken, is ⅞ of an inch in circumference, and contains 35 beads in the space of half an inch; a circumference never, perhaps, before met with, in an entrochi of such a diameter: the medulla is pentagonal. Some specimens are curved, and uniformly serrated on the concave side, fig. 8. pl. XVIII. They are always thicker at one end than at the other, and diminish gradually to a point: the medulla is round. From a fragment, fig. 7. it is probable, that the brachia of the common round entrochi terminated in this variety. Some appear to have been wounded in a recent state, fig. 1: this is indicated by an unusual swelling, which, in every case, exhibits a large and deep puncture, probably the cause of the swelling.
THE Brachia, which are not uniform in their direction, lay a foundation for several varieties. Many of them make acute angles with the stem, fig. 17. pl. XVII. others, right, fig. 18. The greatest number are sparse; but some few are opposite, fig. 18. They penetrate deeper than the cortex, but do not extend to the medulla. Some of them, as in fig. 15. are inserted in one of the beads only; but others extend over a considerable number, fig. 16. The place of their insertion, at a, is large and hollow. Some specimens are found having one end convex, exactly corresponding to the cavity in the stem where it was inserted, and from which it seems to have been torn. I never observed any marks of brachia on the specimens that are curved and serrated. Fig. 13. pl. XVIII. represents a fragment of the Encrinus, the supposed head of the entrochi. The specimen from which it was taken was found, along with entrochi, shells, &c. in till incumbent on lime-stone at Hermyres, and is the only one of the kind I ever saw. The pieces of which it is composed are joined to one another by means of sutures. Many of the quarries contain fragments of a different kind: one of which is delineated fig. 12. It is made up of five-sided pieces, that are smooth on both sides, and about ⅟10 of an inch in thickness: they are concave within, answerable to the convexity without. Great numbers of these pieces are found separately, but seldom conjoined. Two or three whole specimens were all I could find: they are open at top, and commonly full of till. The basis is made up of 5 pieces, of a different shape from the rest; very much resembling the Calex in vegetables. These pieces are so exactly fitted in the lower end as to form a circle, with radii, and a medulla, corresponding to the stem of an entrochi, as in fig. 23. The upper edges of the pieces at the top are furnished with striæ, which must have corresponded with other pieces of a similar construction, probably the fragments represented fig. 14. This fragment, which is always found separately, is raised up like a ridge, in the one side, but hollowed out into a groove, in the other, as in fig. 15. Draughts of detached bases are given fig. 16, 17.
THE specimens, fig. 11. is composed of small pieces, of different sizes. This variety is very scarce; two specimens were all I could find. They are greatly crushed, and have the same appearance on both sides. Originally they were, perhaps, shaped like a ball. Having no remains of entrochi adhering to them, it is uncertain whether they belonged to that genus or not. Several fragments, or single pieces of considerably large dimensions, fig. 10. are sometimes found.
FIG. 9. is a draught of a fragment of what Lhwyd (Lithoph. Brit. No. 1106.) calls Astropodium multijugum, sive Loricatum cinereum septentrionalium. Encrinus cinereus Lachmundi. He speaks of it as a very rare fossil, that was first discovered in England, by William Nicolson, Archdeacon of Carlisle, who found them along with entrochi in lime quarries in Wales. It is a fragment of what Whitehurst calls lilium capidium. It consists of small pieces connected by means of striated articulations, as in fig. 18. which exhibits one of the single pieces. Specimens are found in many of the lime quarries in Kilbride.
FIG. 4, 19, 20, 21, 22. are the delineations of fragments which commonly go under the name of Astropodia. They seem to be parts of the Encrinus, &c. Some of them, as fig. 20, 21. are finely granulated. All the varieties are found, with shells, &c. in most of the lime quarries.
1 St. Cuthbert was the eighth Bishop of Landisferne, about the latter end of the 7th century, and highly famed, in legendary records, for his piety and austerity, when living; and, for miracles performed by his body, when dead.
2 Syst. Nat. p. 1288.