412 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
The two genera, Ceratodus and Ctenodus, were correctly regarded as materially 
differing from the dentition of Cestracion, each tooth occupying a ramus of the 
upper or lower jaw respectively ; those of each jaw being attached to each other at 
the median lire. : 
After a consideration of the microscopical structure of the teeth, they are divided 
into three groups. ‘The first, comprehending the genera Ptychodus, Strophodus and 
Acrodus, of which the existing Cestracion may be taken as the type; it is distin- 
euished by an outer layer of enamel on the dentine forming the surface of the 
crown. The second group is composed of the genera Chomatodus, Cochhodus, 
Psammodus, and Ceratodus, and in this the layer of enamel is absent, and the 
medullary canals open on the surface of the tooth. The third group is formed of 
the two genera Ctenodus and Ctenoptychius; they are characterized by the posses- 
sion of an almost homogeneous surface of dentine which only exhibits occasional 
traces of the medullary canals or tubes. Not having a tooth of Helodus at his 
disposal, Agassiz was unable to assign its position but considered it would probably 
fall into the second group. 
Prof. Owen (“Odontography,” p. 49, et seq.) accepts the classification of Agassiz, 
and regards the existing Cestracion of the Australian seas as the only existing 
representative of the fossil genera indicated by that ichthyologist. Hybodus, Orodus 
and Petalodus are also included in the family. Prof. Owen disagrees with Prof. 
Agassiz as to the microscopical structure of the teeth and contends that the 
medullary tubes have no relation to the punctate impressions of the enamelled 
surface. The “tubes always terminate at a short distance from the surface of the 
tooth, either by anastomosis or by sub-division into other tubes of such extreme 
minuteness that the combined diameters of five hundred of them would barely equal 
the breadth of a single superficial punctation. These impressions on the teeth of 
Psammodi, like the transverse ridges of those of the Ptychodi, are consequences 
of the conformation of the original matrix, and can be regarded only as adaptations 
conformable with the habits and food of the extinct species ; and as they are not 
due to a-certain tubular structure, so neither can they be viewed as evidence of 
such structure when it has not otherwise been proved to exist.” 
With regard to the extinct genus Cochliodus, Prof. Owen remarks that it presents 
avery interesting modification, resembling in structure those of Psammodus. Here 
the jaws are paved with teeth arranged in a few oblique contorted series, as in the 
Cestracion, but a single tooth occupies the space covered by an entire row in the 
existing Australian genus. In the specimen figured (“Odontography,” pl. 222, fig. 1.) 
there are three of these large contorted dental plates in each ramus of the jaw. 
“The microscopic structure of these large teeth closely resembles that of the true 
Psammodus; the medullary canals have the same straight, sub-parallel course, and 
sparing dichotomous sub-divisions, but they are relatively wider, and are separated 
