Psammodus[521] (sandy-tooth). [Pl. VI. fig. 1]; [Lign. 128], fig. 2.—To this genus are referred the fossil teeth of the extinct Cestracionts, which have the crown formed of small vertical tubes, with the grinding surface more or less smooth, and presenting only a punctated or sandy appearance.[522] These teeth are generally flat or slightly convex, and of a square or oblong form; the base is osseous, and as large as the crown. Two species are figured, [Lign. 189], fig. 2, and [Pl. VI. fig. 1a]. A magnified vertical section of the crown, displaying the medullary canals and radiating calcigerous tubes, is represented [Pl. VI. fig. 1b], and a transverse section, fig. 1c; they are thin slices of a tooth, P. porosus, from the Black Rock (Mountain Limestone), near Clifton, viewed by transmitted light. The large, flat, quadrilateral, oblong teeth that abound in the Stonesfield Slate belong to the Strophodus magnus.
[521] See Odontography, pl. xviii. xix.
[522] Ibid. vol. i. p. 59.
There are several kinds of fossil teeth which possess the same essential structure as those of Psammodus, but differ in their external characters; these are referred to other genera by M. Agassiz. Thus Orodus, [Lign. 189], fig. 3, comprises those elongated teeth in which the centre of the crown forms an obtuse transverse cone, traversed by a ridge from which oblique furrows diverge transversely towards the circumference. Similar teeth, but with a smooth, obtusely conical crown, are referred to the genus Helodus. Those with the crown compressed and elevated, with a sharp edge, and with the base surrounded by concentric folds, constitute the type of Chomatodes. A similar crown, but subdivided by deep transverse ridges into dentations, characterises the genus Ctenoptychius.
Ceratodus (horn-tooth) emarginatus. [Lign. 194], fig. 1.—Very curious dental organs, possessing a structure analogous to that of the teeth of Psammodus, are found in the Bone-bed of the Lias; they consist of consolidated plates instead of separate teeth; there was probably but one plate on each side the jaws. The upper margin is generally undulated, and more or less worn by use. These dental plates are composed of two distinct layers; the lowermost portion, or root, is an osseous, reticulated tissue, as in cartilaginous fishes in general; and the upper consists of dentine, with minute parallel vertical tubes, as in Psammodus; these tubes are a continuation of the medullary tissue of the osseous root.
One species occurs in the Great Oolite at Stonesfield, and very many forms abound in the Bone-bed at Aust Cliff, near Westbury on Avon: and in the Trias (bone-bed) of Germany the teeth of several species of Ceratodus are very abundant.
The fishes to which these fossil teeth, referred to Ceratodus, belonged were most probably Cestracionts; the ray-spine known as Nemacanthus is provisionally assigned to them.
Edaphodon. [Lign. 190] and [Lign. 191], Ly. p. 276, fig. 309.—The Chimæroid fishes, though formerly placed with the Plagiostomes (Sharks and Bays), constitute a distinct group, of which there are but two recent genera, though several occur in a fossil state. Their dental organs are very peculiar. Their mandibles are furnished with two or more pairs of oblong teeth, composed of long hollow cylindrical columns, placed nearly at right angles to the grinding surface, which is pitted with minute depressions. These teeth are never shed, but are persistent, and grow on through life, as in the Rodentia, exhibiting in this respect a striking contrast with those of the Sharks, which are feeble and numerous, and constantly replaced by rows of successional teeth.
Fossil teeth of several species, some much larger than the recent, have been found in the Tertiary, Cretaceous, and Oolitic deposits. The first British specimen was discovered in the Chalk-marl at Hamsey, in 1820, by myself; but its nature was not suspected until more perfect examples were obtained from the Kimmeridge Clay at Shotover by Sir P. Egerton, and were submitted to Dr. Buckland, who subsequently ascertained their characters and relations by an examination of the dental organs of a recent Chimæra in the Museum at Leyden in 1835.[523]