XVIII 



CHONDROSTEI 



487 



of the lower jaw are dentigerous. Except for the absence of an 

 interoperculum, the opercular series of bones is complete, includ- 

 ing numerous branchiostegal rays. There is a single small 

 median jugular plate. 



The Palaeoniscidae are remarkable both for their individual 

 and specific abundance and for their extensive range in time. 

 Eepresented only by Cheirolepis in the Middle Old Eed 

 Sandstone and Devonian, the family attained its maximum 

 development in the later Palaeozoic rocks (Carboniferous and 

 Lower Permian), became rare in the Mesozoic, finally dwindling 

 away at the close of the Jurassic period. Their geographical 

 distribution in the past is hardly less remarkable. In various 

 geological formations they have been found in Great Britain and 

 Ireland, in widely remote parts of continental Europe, and in 

 North America, South Africa, and Australia. Cheirolepis, Ambly- 

 pterus, CanoMus, Fhanerosteon, JElonichthys, Crypliiolepis, Palaeo- 

 niscus, and Trissolepis are Palaeozoic genera. Gyrolepis, Urolepis, 

 Coccolepis, Oxygnatlius, and Centrolepis are characteristic Meso- 

 zoic forms. 



Fam. 2. Platysomidae.^ — More or less deep-bodied Fishes, 

 with elongated dorsal and anal fins, a high head, short jaws. 



Fig. 285. — Restoration of Eurynotus crenatus. in.cl, Infra-clavicle ; l.l, lateral line ; 

 orh, orbit ; otter reference letters as in Fig. 284. (From Traquair.) 



usually armed with bluntly conical tritoral teeth, and a complete 

 investment of high, narrow, rhombic scales. They agree with 

 the Palaeoniscidae in their osteology and in most other essential 



1 Traquair, Trans. Roy. Soc. Edini. xxix. 1879, p. 343. 



