If fig. 29 be compared with fig. 27 a, it would seem as if the several teeth of each oblique row in Cestraction had been welded into a single dental mass in Cocidiodus, the proportions and direction of the rows being closely analogous. Whether in Coch2iodus, there were any small anterior prehensile teeth, is hypothetical ; the large crushing dental plates must have been admirably adapted to crack and bruise the shells of mollusks and crus taceans. The Coddiodus contortus (Ag.) (fig. 29) has been found in the carboniferous formations near Bristol and Armagh, and the genus is peculiar to that geological period.
Teeth referable to the genus Hy bodus occur in all the secondary rocks from the trias to the chalk inclusive.
A form of tooth which more closely resembles the crush ing-teeth of Cestracion, is that on which the genus Acrodus is founded, and which also ranges from triassic strata to the upper chalk of Maestricht The species here selected (fig. 30) is the Acrodus nobilis, from the lies of Lyme Regis. The upper figure shows the grinding surface, which, from its finely and transversely striated character and dark colour, has suggested to the quarrymen the name of " fossil leeches." The older fos silists regarded these teeth as petrified Vermes ; but the struc ture, as shown by the microscope, is closely similar to that of the teeth of Cestracion.* Portions of the jaw of the Acrodus
have been discovered which show that these teeth were ar ranged, as in Cestracion, in oblique rows, with at least seven teeth in each row. Acrodus lateralis is a muschelkalk fossil, A. hirudo a Wealden fossil, and A. transversus a cretaceous fossil. No tooth referable to the genus has been found in any tertiary stratum.
The genus Ptychodus is founded on teeth usually of large size, and of a more or less square form (fig. 31). The crown is deeper than the root, which is obtuse and truncate. The enamelled summit of the crown is granulate at the margin, and raised in the middle into an obtuse nence, disposed in large transverse, parallel, sometimes wavy and rather sharp ridges. With teeth of this form are sometimes found others of smaller size, with more convex rounded crowns, doubtless forming the ex tremes of the multiserial pavement which, as in modern sharks and rays, covered the broad jaws of the Ptychodonts. Large dorsal spines have been found so associa ted with the above-described teeth as to indicate the affinity of the Ptychodus to the Cestraciont sharks. All the specimens and species referable to this genus have been found in the cretaceous strata.