The bony plates of these fins are occasionally found lying in irregular groups in the Chalk, as if the fin had partially decomposed and the plates separated. In one example, the rays are split asunder by a piece of bone, apparently a portion of a long pointed tooth, firmly impacted between them; as if the fish had been seized by some enemy, and had escaped, with the tooth of its adversary in its fin. Very fine specimens have been found at Charing, Kent, by W. Harris, Esq. F.G.S.
In the fragment of an Ichthyodorulite from the Lewes Chalk, a remarkable structure is displayed; the osseous plates are united laterally by smooth, longitudinal lines, as in those above described; but they are also traversed by numerous oblique, finely-serrated sutures. [Lign. 188], fig. 1a.
The Chalk contains rays of other species of Ptychodus, as well as of some allied genera. Of these, the most remarkable are smooth, arched, pointed spines, having a shallow posterior groove, with an enamelled surface, marked with fine longitudinal striæ, and frequent, parallel, oblique lines. These, according to Sir P. Egerton, belong to a true Cestracion (see [p. 584]): they were first figured and described by me (Foss. South D. tab. xxxiii. fig. 5) as belonging to the Acanthias major, and were subsequently assigned to the genus Spinax by Prof. Agassiz (Poiss. Foss. iii. p. 62).
It may be necessary to remark, that the fins first described have been referred to the fishes which yielded the large grooved teeth so common in the Chalk (see [Pl. VI fig. 2]) in consequence of their affinity to existing species, which have similar fins and teeth; and from the circumstance that the Sharks of the genus Lamna, whose teeth also abound in the Chalk, have no dorsal rays of this kind; still the proof of identity remains to be discovered. In one specimen only have I observed indications of any other part of the skeleton; it is a spine of Acanthias major, the base of which rests on several dorsal vertebræ (Foss. South D. tab. xxxiii.).
Hybodus subcarinatus. [Lign. 188], fig. 3.—The fishes of another extinct genus of Sharks, termed Hybodus, from the gibbous form of the teeth, were also provided with dorsal spines, which may be readily distinguished from the preceding. These Ichthyodorulites are generally long, slightly arched, and terminate in a point at the extremity; the base, which was implanted in the flesh, is deeply grooved, and much prolonged, being sometimes equal to one-third of the entire length. The surface is marked with strong longitudinal ridges, parallel with the anterior margin which is rounded and laterally compressed. The posterior edge, which is more or less flat, has, towards the base two rows of sharp arched teeth, which gradually approach ’each other, and blend into one line on the upper part of the ray There are numerous species of this genus in the Oolite and Lias. I have found one species in the Chalk and a few in the Wealden. The small Ray figured [Lign. 188] fig. 3, is from Tilgate Forest, and displays the usual characters of these fossils. From specimens discovered in the Lias, associated with the teeth, it appears that the Hybodus had two dorsal fins, each furnished with rays, as in the recent Dog-fish.
The microscopic structure of these rays is stated by M. Agassiz to resemble that of the teeth: in some there is a pulp cavity, which occupies the centre of the spine, and is surrounded by dentine, in which the calcigerous tubes radiate direct to the surface; the external enamel is a layer of dentine, in which the medullary canals are wanting.
In the strata below the Lias there are numerous Ichthyodorulites, some of a large size, belonging chiefly to the Cestracion family, and of extinct species, not observed in more recent deposits. Thus there are several species of dorsal rays (named Onchus, from their hooked form,) that are wide at the base, and bent backwards, with the posterior margin destitute of teeth, in the Carboniferous, Devonian, and Silurian formations; also immense compressed spines, having small teeth on the posterior margin, and the surface covered with longitudinal striæ, and finely toothed, transversely; hence termed Ctenacanthus, or pectinated-spine (Murch. Sil. Syst. p. 596).
The fossil spine, named Orthacanthus (Poiss. Foss. vol. iii. pl. xlv.), and found in the Coal of Manchester, has been discovered in connexion with the body of the fish to which it belonged in the Carboniferous deposits of Ruppersdorf in Bohemia (Geol. Journal, vol. v. part ii. p. 23).
Some Ichthyodorulites have the surface richly ornamented with stellular tubercles, and are termed Asteracanthus, or starry-spine; there are very large fin-rays of this kind in the Wealden, Purbeck, Oolites, and Lias.[517]
[517] For particular information on Ichthyodorulites, consult Poiss. Foss. tom. iii. chap. i. About seventy species are enumerated.