146 THE ANATOMY OF VERTEBRATED ANIMALS. 



in Polypterus — sometimes greatly elongated and almost fili- 

 form, as in Holoptychius (Fig. 43). There are no branchio- 

 stegal rays, biat two principal, and sometimes many smaller 

 lateral, jugular plates. The tail may be either diphycercal or 

 heterocercal. 



The only living representatives of this suborder are 

 Folypterus and Calamoichthys, which inhabit the rivers of 

 North Africa. Neither of these are known to occur in the 

 f(jssil state. The only family of the suborder at present 

 known among Mesozoic fossils is that of the CcBlacanthini, 

 a remarkable gi-oup of fishes with a persistent notochord, 

 rudimentary ribs, an air-bladder with ossified walls, and a 

 single interspinous bone for each of the two dorsal fins. 

 The Ccelacanthini also occur in the Carboniferous forma- 

 tion; and the great majority of the Crossopterycjidoe are 

 found in this and the Devonian formations (Osteolepis, 

 Biploptenis, Glyptolcemus, Megalichthys, Soloptijchius, Bhi- 

 zodus, Dipterus, Fhaneropleuron. &c,). Megalichthys. Di- 

 pterus, and probably a few other of these fishes, have partially 

 ossified vertebral centra; the i-est possessed a persistent 

 notochord. It is by the Crossopterygidce that the Ganoids 

 are especially connected with the Jiipnoi, and, through 

 them, with the Amphibia. 



4. The Chondrosteidce are either naked, or have dermal 

 plates of bone in the place of scales. Neither the pectoral 

 nor the ventral fins are lobate. The branchiostegal rays 

 are few or absent, the tail is heterocercal. There are no 

 <;artilage bones in the brain-case, The teeth are very small, 

 or absent. 



The Sturgeons [Accijie^iser) — which inhabit the northern 

 rivers of Europe, Asia, and America, occasionally migrating 

 to the sea — Spatularia, and Scapirhyiichus (f aimd in the rivers 

 of North America), are the recent members of this group, 

 which is represented, in the older Mesozoic rocks, by Chon- 

 drosteus. 



5. The CephalaspidiB are remarkable fishes, probably 

 allied to the Chondrosteidce. which occur only in the Lower 

 Devonian and the Upper Silurian rocks, and are some of 





