[519] The microscopical structure of the teeth of Acrodus is well shown in the "Odontography," pl. xiv. xv., and beautifully illustrates the relation of dentine to bone.
Lign. 189. Fossil Teeth of Sharks.
| Fig. | 1.— | Ptychodus Mortoni. (G. A. M.) Cret. New Jersey. |
| 2.— | Psammodus cinctus. (Ag.) Mt. L. Bristol. | |
| 3.— | Orodus cinctus. (Ag.) Mt. L. Bristol. | |
| 4.— | Acrodus nobilis. Lias. Lyme Regis. |
Ptychodus (wrinkle-tooth). [Pl. VI. fig. 2]; [Lign. 189], and [Lign. 191].—The palatal teeth, which occur more or less abundantly in almost every chalk-pit, and are known by the name of "palates," belong to several species of the genus Ptychodus. A very common form is figured [Pl. VI. fig. 2]; and microscopic views of vertical and transverse sections, as seen by transmitted light, are shown in figs. 2b, 2c. Groups of these teeth, somewhat naturally arranged, and varying in size and form according to the situations they occupied in the jaws, are occasionally found: one specimen in the British Museum, and formerly in my collection, contains more than 120 teeth. In general they occur in a very perfect state, with the osseous base and enamelled crown entire. The dorsal rays or spines previously described ([p. 577]), are sometimes found with the teeth, and belong to fishes of the same genus.
These teeth are of an angular form, and more or less square, the crown is wider than the root, which is obtuse, truncated, and depressed in the centre; the enamelled part of the tooth is expanded at the edges, and forms in the centre a flattened or slightly convex mammillary projection, which is traversed by large, acute, transverse, parallel ridges. The borders are granulated, and the sides of the projection marked with deep vertical plicæ or folds; this description particularly applies to the species named P. polygurus, figured in [Plate VI.] Dr. Buckland has represented a fine group of these teeth, Bd. ii. pl. xxvi′. Another common species (P. decurrens) is distinguished from the former by the connexion between the large furrows on the crown and the granulations on the expanded border, which diverge from the outer edge of the large folds to the margin of the enamel.
The microscopic structure of these teeth presents the same congeries of medullary and calcigerous tubes as those of the recent Cestracion: see [Plate VI. figs. 2b, 2c].
The teeth of a species of Ptychodus occur in the arenaceous strata of the Chalk-formation in New Jersey, which possess the essential characters of the European types, but differ from them in their configuration; the only specimen I have seen is figured [Lign. 189], fig. 1; it was presented to me by Dr. Morton. The enamelled crown forms a conical projection, traversed by large inosculating ridges, which radiate from the summit towards the margin.[520]
[520] I have named it P. Mortoni, in honour of my distinguished friend, the eminent American naturalist and physician, Dr. George Morton, of Philadelphia, by whom it was discovered.