214 



HISTORICAL GEOLOGY 



dominant Paleozoic types were gone and all were forms of regular 

 shape. For the first time the Echinoids were more prominent than 

 the Crinoids. 



Molluscoids. — Bryozoans continued, but with certain impor- 

 tant genera changes. 



Brachiopods showed two important changes, namely (1) a great 

 reduction in number of species and of individuals, and (2) the shells 



with straight-hinge lines becoming sub- 

 ordinate to those with curved-hinge lines 

 for the first time, a common genus (Tere- 

 bratula) of the latter being represented 

 by a Cretaceous form in Fig. 159. To 

 the present day the Brachiopods never 

 again became conspicuous elements of the 

 fauna. In spite of the important changes, 

 a few of the Paleozoic genera survived the 

 transition to the Mesozoic. 



Mollusks. — This subkingdom included 

 all of the most abundant invertebrate 

 animals of the period, all of the well- 

 known classes having been prominently 

 represented. 



Pelecypods were more numerous and 

 diversified than ever before. They vastly 

 outnumbered the Brachiopod bivalves. 

 Certain still existing genera were intro- 

 duced so that many forms were of decided modern appearance. 

 Gastropods. Several Paleozoic genera existed for the last 

 time, and certain more modern types appeared. 



Cephalopods. Among the Nautiloids the straight-shelled 

 form (Orthoceras) , known from the very early Paleozoic, became 

 extinct in the Triassic, while the coiled forms were still common and 

 much like those of later Paleozoic time. 



Among the Ammonoids an evolutionary feature of particular 

 interest was the development of still greater complexity of shell 

 structure. Goniatite-like forms still persisted, but, even early in 

 the Triassic, forms with slightly serrated sutures or partition 

 structures (e.g. Ceratites, F ig. 127) appeared. Later in the period 

 representatives of the most complex of all known chambered 

 Cephalopods, that is the Ammonites, appeared (see Fig. 141). 



Fig. 127 

 A Triassic Ceratite, Cera- 

 tites trojanus, with part 

 of shell removed to show 

 suture structure. (After 

 J. P. Smith, slightly 

 modified by accentua- 

 tion of sutures.) 



