60 



PISCES. 



.directly over the pectoral arch, is laterally compressed, smooth or longi- 

 tudinally striated, and with a double series of posterior denticles. The 

 dental plates are all very massive, but the symphysial union of those of 

 the mandible is narrow. The number and disposition of the tritors on 

 the palatine and mandibular plates are almost always as represented in 

 fig. 46, no. 2, and fig. 47, nos. 1, 2. Ischyodus ranges from the Lower 

 Oolite to the Lower Chalk; and each mandibular tooth in the type 

 species, /. townsendi (from the English Portlandian), sometimes measures 

 as much as O014 m. from its symphysial border to the extremity of its 

 post-oral margin. 



FIG. 47. 



Diagram of the inner aspect of the right mandibular tooth in the principal genera 

 of Chimseridffl, showing the arrangement of the tritors and the extent of 

 the symphysis. 1, 2, Ganodus and Ischyodus; 3, Edaphodon; 4, Callo- 

 rhynchus; 5, Elasmodus; 6, Chimcera; 7, Elasmodectes. (From Brit. Mug. 

 Catal.) 



Sub-Class 3. Dipnoi. 



The Dipnoan fishes formed a most conspicuous feature in 

 the Palaeozoic vertebrate fauna, and did not become entirely 

 insignificant until the period of the Muschelkalk (Trias), after 

 which they seem to have almost, if not quite deserted the sea. 

 Our present knowledge of the facts also suggests, that the 

 Devonian period witnessed the maximum of their development 

 and specialization ; one order which seems to be referable to 

 this sub-class comprising great armoured fishes which are no 

 longer met with above the lowermost Carboniferous. The 

 Dipnoi have always been destitute of calcifications in the sheath 

 of the persistent notochord ; the membrane-bones of the cranial 



