490 On the Fossil Hishes of the Carboniferous Limestone Series of Great Britain. 
purpose of crushing and masttcating the food of the fish. The teeth of Janassa, 
formed not only a sharp cutting edge, but a large palatal surface extending 
horizontally backwards along the floor and roof of the mouth. Those of 
Petalorhynchus differed materially from this arrangement and constituted only a 
sharp cutting surface; they appear to have been the teeth of a predaceous fish, 
rather than of one which was a vegetable feeder as Messrs. Hancock and Atthey 
argue that Janassa was. 
Prof. Agassiz, who described and figuredthe Janassa of Miinster as Acrodus larva 
(“ Pois. Foss.,” Vol. III, p. 147, tab. 22, figs. 25-25), but afterwards recognised its 
proper position and affinities (“‘ Op. Cit.,” p. 376), considered that Janassa was an 
intermediate type between Myliobates and the Cestraciontes, and contributed to 
bridge over the gap between the rays and sharks, the limits of which appear 
always less sharp in proportion as they become better known. Prof. Agassiz also 
points out certain analogies between Janassa and Zygobates and Rhinoptera. 
The dentition of the genera Myliobates and Zygobates possess several characters 
in common. In place of pointed teeth they possess a series of dental plates, which 
form a pavement-like covering attached to each upper and lower jaw: the teeth 
are attached to each other by sutures, and extend in rows of seven teeth each, 
from the front of the mouth far backwards towards the gullet. The chief 
peculiarity of these teeth lies in the large development of the central plate, which 
extends across the symphysis of each jaw ; it is larger in Myliobates than all the 
remaining six teeth together. The latter are arranged three on each lateral 
extremity of the central one. The central teeth of Zygobates are proportionately 
not so large as those of Myliobates, the side teeth diminishing im size as they 
recede from the central one. Both genera are represented by a few existing fishes 
but are found fossil in large numbers amongst the Tertiary rocks. The teeth of 
Janassa and Petalorhynchus are similar to, and agree with those of Myliobates and 
Zygobates in the arrangement of the teeth in transverse rows, and in the position 
of the central or medium tooth which covers the symphysis of the Jaw: a 
peculiarity which does not ordinarily obtain in the dentition of fishes, the teeth 
usually occupying positions on, or in each ramus of the jaw but not extending 
across, or otherwise connecting, the two sami. The latter feature, especially 
considering its rarity, is remarkable, and it was from its occurrence in Janassa that 
Prof. Agassiz derived the idea that the genus possessed affinities with the rays 
How far he may be correct has not yet been demonstrated. The broad, ridged 
teeth of Janassa with short horizontal crowns, adapted both for cutting and 
crushing its food, bear a greater resemblance to the flat pavement teeth of 
Myliobates, adapted only as a crushing or triturating surface, than do those of 
Petalorhynchus. The teeth of the latter are pre-eminently cutting teeth, and 
though a number of imbricated folds or ridges usually extend across them, it is not 
probable that they were of the least service for masticatory purposes, the teeth 
