THE JURASSIC PERIOD 



229 



Echinoderms. — After their culmina- 

 tion in the Mississippian, the Crinoids 

 remained in a comparatively subordi- 

 nate position during the Permian and 

 Triassic periods. During the Jurassic 

 they again became profuse. As regards 

 both abundance and size they probably 

 even surpassed those of the Mississip- 

 pian, though not in diversity of species. 

 Their general structure was more like 

 modern forms than like Paleozoic, and 

 also there is good evidence that the 

 shallow-water forms so common in the 

 Paleozoic began to give way to deeper- 

 water forms similar to those so prevalent 

 today. Fig. 138 gives a good idea of one 

 of the Jurassic Crinoids, the highly seg- 

 mented and delicately branching arms 

 being well exhibited. It scarcely seems 

 credible that fully 600,000 segments 

 have been counted in a single individual. 



Aster ozoans were moderately represented and they had already 

 assumed a distinctly modern structure. 



Echinoids. — These forms, which first 

 attained much prominence in the Triassic, 

 continued to increase in abundance and 

 variety in the Jurassic. Early in the period 

 regular forms only existed, but later in the 

 period the irregular forms made their first 

 appearance. The regular forms were radi- 

 ally symmetrical, while the irregular ones 

 were only bilaterally symmetrical (see Figs. 

 139, 140). Since the latter are distinctly 

 more modern in structure, we have here 

 another good illustration of progressive 

 evolution toward modern forms. 



Molluscoids. — Bryozoans were present, 

 but apparently not very important. 



Brachiopods were still fairly common, 

 though the numbers of genera and species 



Fig. 138 

 A Jurassic Crinoid, Pen- 

 tacrinus fossilis. (After 

 Goldfuss.) 



Fig. 139 

 A regular or radially 

 symmetrical Echi- 

 noid, Pseudodiadema 

 texanum, of Lower 

 Cretaceous age. (Af- 

 ter Hill and Vaughn, 

 U. S. Geological Sur- 

 vey, Folio 76.) 



