SUCCESSION OF FOSSIL PAL&ICHTHES. 



503 



the Oolitic Thaumas belong to this type. Squaloraja of the Lias 

 represents the living Pristiophorus. The Rays are represented by the 

 saws of Pristis in the London clay and newer rocks. Spathobatis 



Dorsal spine. 



Fig. 137.- 



Tooth. 



-Hvbodus. 



is a ray from the Oolites. The torpedoes appear in the genus Cyclo- 

 batis in the Cretaceous of Lebanon, though the genus Torpedo is first 

 found in Monte Bolca. Arthropterus from the Lias was a true ray, 

 and ray spines are met with in the Coal and in Crag. The sting- 

 rays are numerous in the Lower Tertiary ; the eagle ray Myliobatis, 

 with the allied genera ^Etobatis and Rhinoptera, characterise the 

 Lower Tertiary, and Zygobatis is found in the Crag. The Chimaeras 

 appear in the Devonian, according to Dr. Newberry, with Rhynclw- 

 dus. Ganodus is found in the Lower Oolites. Ischyodus ranges from 

 the Lias to the Chalk. Edaphodon is Cretaceous, but is represented 

 in the Tertiary. Elasmodus and Psaliodus are Tertiary. Callorhyn- 

 chus, a living type, is found in the Cretaceous of New Zealand. 



The Ganoid fishes have been divided into eight sub-orders. The 

 Placodermi are extinct ; they range from the Lower Ludlow to the 

 Carboniferous, but are characteristic of the Old Red Sandstone, and 

 comprise Ptericlithys, Coccosteus, Dinichthys, Cephalaspis, and the 

 allied forms Pteraspis, Scaphaspis, Auchenaspis, &c. Another sub- 

 order, distinguished by carrying large spines, is represented in the Old 

 Red Sandstone and Carboniferous rocks by Acantliodes and Cheira- 

 canthus. The Dipnoi are represented by the existing Ceratodus, which 

 is found in the Trias and Lower Oolites, but is unknown in newer 

 strata. Closely allied are the extinct Dipterus and Heliopus of the 

 Devonian ; Plianeropleuron belongs to the same rocks. The sturgeons 

 are not known prior to the London Clay, though Polyodon is repre- 

 sented in the Lias by Chondrosteus. The Polypteroid fishes comprise 



Fig. 138. Polypterns (living), 

 a. pectoral fin ; 6. ventral fin ; c. anal fin. 



the Saurodiptemni, including the Old Red Sandstone Osteolopis and 

 Diplopterus, and the Carboniferous Megalichthys. The Coelacanthi 



