ORDERS OF FISHES. 



139 



boidal, and are articulated to one another by processes 

 developed on the internal surface of their anterior margins. 

 In the genus Platysomus (fig. 505) the tail is heterocercal, 

 the dorsal and anal fins are long, the pectorals are small, 

 and the ventrals appear to be wanting. The teeth are 

 conical and uniserial, and the body is deep and compressed 



Fig. 505. — Platysomus gibbosus. Middle Permian. 



from side to side. The Platysomi are mainly found in the 

 Permian rocks. Another genus of this family is the Car- 

 boniferous Cheirodus of M'Coy ( = Amphicentrum), in which 

 the body is deep and rhombic ; the scales are high and 

 narrow ; and the front of the jaws is edentulous, while 

 the palate and hinder portion of the mandible are furnished 

 with ridges carrying " small tubercular tooth - like eleva- 

 tions " (Traquair). The other genera included by Traquair 

 in the family of the Platysomidce are Eiirynotus, Bencdenius, 

 Mesolepis, Eurynomus, and Wardichthys. 



SuB-ORDEK D. CPtOSSOPTERYGiDiE. — " Dorsal fins two, or, 

 if single, multifid or very long ; the pectoral, and usually the 

 ventral, fins lobate ; no branchiostegal rays, but two principal, 

 with sometimes lateral and median, jugular plates, situated 

 between the rami of the mandible ; caudal fin diphycercal, 

 or heterocercal ; scales cycloid or rhomboid, smooth or sculp- 

 tured." — (Huxley. ) 



All the Ganoids of this sub-order are pre-eminently dis- 

 tinguished by the structure of the paired fins, the pectorals 

 always, and the ventrals usually, consisting of a central lobe 



