492 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
characters which appear to ally them with the more recent genera Mylhiobates and’ 
Zygobates of the Tertiary formations, whose representatives still exist, principally 
inhabiting tropical seas. The fossil remains of the several genera are so 
fragmentary, that it is extremely difficult to obtain sufficient data whereon to base 
deductions of so great import as those above indicated. Of the older genera 
Petalodus and Petalorhynchus absolutely nothing has been found except the teeth ; 
and those only, except in very rare instances, as detached specimens. Along with 
the teeth of Climaxodus and Janassa, masses of minute tuberculous shagreen have 
been found associated. In a variety of the latter genus, described by Miinster as 
Dictea striata (Beitrige zur Petrefactenkunde, tome iii., tabs. 3 and 4, fig. 1), the 
form of the whole body is indicated and is completely covered with fine shagreen. 
The length of the fish is 0°390 metre, exclusive of the tail, the greatest breadth is 
0:110 metre between the pectoral and ventral fins, the height of the head is 
0:080 metre. The specimens obtained from the Marl Slates of Durham have also 
associated with them masses of shagreen, and from the arrangement of the teeth, 
it is inferred that the mouth was underneath the head as in the Elasmobranchi, 
They possess an anal fin. The more recent genera Myliobates and Zygobates are 
well known from their living representatives. The dentition forms a perfectly flat 
mosaic-like pavement. The body is widely expanded, short, and comparatively 
thin, and does not possess anal fins. It is more or less covered with tuberculated 
shagreen. Occasionally a serrated spine exists in connexion with the dorsal fin. 
The gradual change in the form of the teeth ; from the deeply-rooted, strong 
sharp cutting teeth of Petalodus through various modifications in intermediate 
genera to those of Janassa which combine the sharp cutting edge with a large- 
expansion of the base of the crown in the form of imbricated folds, adapted to 
crushing alimentary substances, may have developed still further to form the 
altogether flat teeth of the Myliobate rays, in which the cutting teeth have entirely 
disappeared. The internal framework of all the fossil genera was composed of 
cartilage and has entirely disappeared. 
Genus.—Petalodus. Owen. 
Ref. R. Owen, 1840. ‘Odontography,” p. 61. 
‘Teeth transversely elongated, much compressed, thin, petal-shaped, cutting edge. 
serrated ; base of crown with several narrow imbricating folds of enamel, descending 
lower on the posterior than the anterior face ; root large, oblong, thin, truncated. 
below ; lower edge, obtuse, tumid.” 
