418 On the Fossil Fishes of the Carboniferous Limestone Series o¢ Great Britain. 
“in place of having a great number of rows of teeth placed the one behind the 
other, in the form of large plaques more or less arched, as im the genera Acrodus 
and Phychodus, or in the form of a cushion of twisted spindles, as in the genus 
Cestracion, exhibits only a small number of teeth disposed in such a manner as to 
cover a large portion of the rounded surface of the dentary border of the jaw.” The 
fragment referred to exhibits a range of three teeth on each; it has, however, the 
appearance of having had a fourth, which is lost. Professor Agassiz further 
considers it evident that the small number of teeth preserved in Cochliodus, take 
an intermediate place between the numerous ranges of teeth which form the fusiform 
palates of the genus Cestracion, and that of Ceratodus with its four teeth, one to 
each ramus of the jaw, and that their large contorted surfaces embraced a large 
space upon the jaw. 
The relationship surmised by Professor Agassiz to exist between Ceratodus and 
Cochliodus may possibly prove correct, but the discovery of an existing species 
of Ceratodus in the Australian rivers by Dr. Krefft, in 1870, has proved that 
should such relationship be found between the two, Ceratodus will not join 
Cochliodus as one of a family of sharks, but that the opinions hitherto held 
respecting the Selachian attributes of the Cochliodonts will have to be seriously 
modified. There are some points of resemblance between the Cochliodonts and Cer- 
atodus which it may be useful to point out. The skull of Ceratodus is primarily 
composed of cartilage, with a few ossified centres. The vertebral column is 
notochordal. The pectoral arch mainly cartilaginous, and the pelvic arch entirely 
so. Twenty-seven pairs of bony ribs. The mandibles are connected with the skull 
by a suspensorium, they are short, with a wide symphysis. The disposition of the 
teeth of Ceratodus is ascertained to be as follows :—“ An osseous palato-pterygoid 
arch supports two palatal dental plates, whose horns or denticles project outwards, 
the larger and more prominent being anterior. In advance of these are two cutting 
teeth imbedded in the vomerine (mesethmoid) cartilage. The mandible supports a 
similar pair of dental plates. These are attached by their imner edges to the | 
splenial bone, which invests the inner side of the jaw, while on the outer side they 
overlie the cartilaginous centre of the ramus, and to some extent the angular bone, 
with which they have a fibrous connection. In biting, the ridges of the dental 
plates interlock, though the opposed surfaces are not so accurately adjusted as 
altogether to prevent a sliding movement ; the vomerine teeth are unopposed. All 
the dental plates are anchylosed to the supporting bones,” (L. C. Miall, “ Sirenoid 
and Crossopterygian Ganoids,” p. 20, Palzeont. Soc., 1878). 
With the present knowledge of the Cochliodonts it appears probable that the 
dentition of some of the genera consisted of a single dental plate to each ramus of 
the jaws, and though the palate and mandibles were cartilaginous with partial 
ossification, the teeth were firmly implanted if not anchylosed. The dental plates 
in Ceratodus rise into external folds or ridges, the surface of the investing dentine 
