528 



HISTORICAL GEOLOGY 



horn-shaped valve. These irregular, unsymmetrical bivalves are 

 usually firmly attached by one valve, their irregular development 

 being due, to some degree, probably largely, to this fact. It is 

 interesting to note that these extraordinary forms appear contem- 

 poraneously with the extravagantly modified cephalopods. A char- 

 acteristic Cretaceous genus is Inoceramus (Fig. 491 Z)), also found 

 in Jurassic deposits. 



Gastropods. — The Mesozoic gastropods (Fig. 492 A, B) were, as 

 a whole, less simple than those of the Paleozoic, although many of 

 the older type, in which the mouth of the shell is a complete ring, 



lived on. In one branch, a tube 

 was developed through which the 

 waste waters of the body were car- 

 ried and emptied some distance from 

 the opening into which the fresh 

 waters entered, a structure the ad- 

 vantage of which is obvious. Forms 

 of this type were lacking in the Paleo- 

 zoic, but became common before the 

 close of the Mesozoic. Towards the 

 close of the era many of the genera 

 which reached their highest develop- 

 ment in the Tertiary and recent times 

 appeared. 



Cephalopods. — The Paleozoic types of cephalopods (Fig. 407 

 A-D) are represented in the Triassic strata by orthoceratites and 

 goniatites and occur with the fringe-sutured ceratites (Fig. 493 I) 

 and the complex-sutured ammonites (Fig. 493 A), but soon disappear. 

 Ammonites. — Ammonites " developed with wonderful rapidity 

 from the first rare members [in the Upper Silurian or Devonian] 

 into numerous families, hundreds of genera, and thousands of species, 

 reaching their acme in the Jurassic." " In the Cretaceous they 

 gradually declined, dropping off one at a time, until all are gone 

 before the end." (J. Perrin Smith.) In numbers, diversity of form, and 

 ornamentation, ammonites are remarkable. Especially towards the 

 end of the race (in the Upper Cretaceous) unusual forms appeared. 

 At this time — and occasionally in the Triassic and Jurassic — many 

 began to uncoil; some were coiled during the early part of their 

 life, but as they approached old age became less coiled (Scaphites, 

 * '£• 493 L) ; others formed open coils (Crioceras, Fig. 494 A) ; some 



Fig. 492. — Mesozoic gastropods 



A, Anchura americana (Cretaceous) 



B, Pyropsis bairdi (Cretaceous). 



