128 THE ANATOMY OF VERTEBRATED ANIMALS. 



jugular plates. The tail may be either dipliyceroal or hetero 

 cereal. 



The only living representatives of this sub-order are Polyp- 

 terns and Calamoichthys, vs^hich inhabit the rivers of North 

 Africa. Neither of these are known to occur in the fossil state. 

 The only family of the sub-order at present known among 

 Mesozoic fossils is that of the Gailacanthini, a remarkable 

 group of fishes with a persistent notocbord, rudimentary ribs, 

 an air-bladder with ossified walls, and a single interspinous 

 bone for each of the two dorsal fins. The Ccdacanthini also 

 occur in the Carboniferous formation ; and the great majority 

 of the Grossopteryijldm are found in this and the Devonian 

 formations ( Osteolcjns, Diploi^erus, Glyptolcemus, MegalicJi- 

 thys, IloJojityrldus, Rhizodus, Dipterus, Phci7ieropleuro7i, etc.). 

 3Iegalichthys, Diptenis, and probably a few other of these 

 fishes, have partially ossified vertebral centra ; the rest pos- 

 sessed a persistent notochord. It is by the Grossopterygidm 

 that the Ganoids are especially connected with the Dipnoi, 

 and, through them, with the Amphibia. 



4. The Ghondrosteidce are either naked, or have derma! 

 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 cartilage-bones in 

 the brain-case. The teeth are very small, or absent. 



The Sturgeons {Aceipenser) — which inhabit the northern 

 rivers of Europe, Asia, and America, occasionally migrating 

 to the sea — Spatularia, Aud Scapirhynchus (found in the rivers 

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

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

 drosteus. 



5. The GephalaspidoB are remarkable fishes, probably allied 

 to the Cho7idrosteida3, which occur only in the Lower Devo- 

 nian and the Upper Silurian rocks, and are some of the oldest 

 fish at present known. The head is covered by a continuous 

 shield, which has the structure of true bone, in Cephalaspis, 

 but more resembles certain piscine scales, in Pterapsis. The 

 shield is prolonged into two horns at its posterolateral angles, 

 and a median dorsal backward prolongation usually bears a 

 spine, in Gephalaspis ; the body is covered with flat bony 

 scales or plates, and possesses two large pectoral fins. The 

 characters of the body and fins of Pterapsis are imknown. 

 Notwithstanding the excellent preservation of many of the 

 specimens of these fishes, they have, as yet, yielded no evi- 

 dence of jaws or teeth. Should jaws be absent, the Gepho 



