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. 
Represented only by Cheirolepis in the Middle Old Red 
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, Canobius, Phanerosteon, Elonichthys, Cryphiolepis, Palaeo- 
niscus, and T'rissolepis are Palaeozoic genera. Gyrolepis, Urolepis, 
Coccolepis, Oxygnathus, 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, 
is 
‘3 
sees 
{3h 
Fic. 285.—Restoration of Eurynotus crenatus. in.cl, Infra-clavicle ; 7.2, lateral line ; 
orb, orbit ; other 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, Zrans. Roy. Soc. Edinb, xxix. 1879, p. 348. 
