26 



HISTORICAL GEOLOGY 



rapid development early in the Mesozoic, became abundant in the 

 Jurassic and Cretaceous, assuming the place formerly held by the 



crinoids, and finally cul- 

 minated in the Tertiary. 

 Starfish. — Starfish are 

 not a conspicuous race 

 in the Mesozoic. 



Brachiopods. — Aside 

 from the abundance of a 

 few genera at different 

 times in the Triassic and 

 Jurassic, there is little of 

 interest in this class in 

 the Mesozoic. Most of 

 the species (Fig. 490 A— 

 D) belong to the genera 

 that are living in the 

 seas of to-day and are 

 almost exclusively of 

 three families. A few 

 of the Paleozoic long- 

 hinged type (Spiriferina, 

 etc.) survived for a short time, but were inconspicuous. After their 

 long period of ascendancy in the Paleozoic, brachiopods became 

 unimportant and have remained in a subordinate position ever since. 

 Pelecypods. — Almost in proportion as the brachiopods declined 

 the pelecypods (Fig. 491 A- J) increased, both in numbers and 

 variety. This rapid development is shown in the Triassic, where only 

 about one fourth were of 

 Paleozoic genera. In the Ju- 

 rassic, the oyster tribe is con- 

 spicuous and is represented 

 not only by the true oyster 

 but by others that, although 

 belonging to the same order, 

 have a different external ap- 

 pearance (Gryphaea, Fig. 491 

 C, Exogyra, etc.) and are much more characteristic. In the Jurassic 

 and still more in the Cretaceous, a number of pelecypod genera 

 appear which depart radically from the typical forms in which the 



Fig. 489. — Mesozoic echinoderms : A, Cidaris 

 coronata (Jurassic) ; B, Cassidulus subconicus (Upper 

 Cretaceous) ; C, Diplopodia texanum (Lower Creta- 

 ceous). 



BCD 



Mesozoic brachiopods : A, Tera- 

 bratula humboldtensis (Triassic) ; B, Rhyncho- 

 nella cequiplicata (Triassic) ; C, Rhynchonella 

 gnathophora (Jurassic) ; D, Lingula brevirostra 

 (Jurassic). 



