TRIASSIC PERIOD 75 



relatives in Triassic seas were probably the straight-shelled 

 nautiloids, rapidly passing away (Ortkoceras). 



The Permian promise if promise it was of the appear- CRUSTACEANS 

 ance of crabs was not fulfilled. Crustaceans, however, were 

 now in view with much nearer resemblance to prawns and 

 lobsters than were the " pod-shrimps " and opossum-shrimps 

 of earlier times (Pemphix). 



King-crabs called crabs by courtesy were now in exist- KING-CRABS 

 ence with a second shield developed. These animals appear 

 to have been identical in structure with the modern King- 

 crab (Limulus]. The half -formed creatures, with front-shields 

 only, seem by this time to have died out. 



Triassic King-crabs were diminutive creatures ; but the 

 departure of their ancestors from the doomed and afterwards 

 overthrown trilobite ranks had ensured survival. 



Fish-life, different though its aspect from that of modern 

 times, was making headway. 



Sharks, after serious reverses in Permian seas, were now SHARKS 

 abroad in new genera, and in great abundance (Hybodus, 

 Acrodus, Strophodus). And less predaceous fishes were enjoy- 

 ing prosperity, not only in spite of old foes, but in the face 

 of new dangers which will be noted presently. 



The old " fringe-fanned " ganoids with paddle-like fins GANOIDS 

 were still waning (Crossopterygii). This Order had apparently 

 played its part and a great one in giving rise to primitive 

 amphibians. 



Among the more flexibly finned ganoids (Actinopterygii) 

 old and new genera were now to be seen. The most notable 

 absentees were certain deep-bodied forms such as had 

 flourished in Carboniferous and Permian seas (Eurynotus, 

 Platysomus). Among new forms may be mentioned some 

 gristly skeletoned fishes with hard enamelled scales (Catop- 

 terus, Dictyopyge), and akin to some fishes of Permian times 

 (Palceoniscus, etc.). They, however, showed advance in 

 development over their forerunners in having the dorsal and 

 anal fins more effectively supported. Their tails, moreover, 

 were of less antique character, in being to a certain extent 

 free of the spinal column. In some closely allied pike-shaped 

 forms an ossification of the spine was well in progress ; 



