52 GEOLOGY. 



ably independent of bottom conditions, but because they were steadily 

 and rapidly advancing in organization, and because their shells were 

 SO constituted as to delicately record their progress by reason of the 

 marvellously complex sinuosities of the sutures, and by the peculiar 

 registration of their life history. " The Ammonoidea preserve in each 

 individual a complete record of their larval and adolescent history, 

 the protoconch and early chambers being enveloped and protected 

 by later stages of the shell; and by breaking off the outer chambers, 

 the naturalist can in effect cause the shell to repeat its life-history 

 in inverse order, for each stage of growth represents some extinct 

 ancestral genus. These genera appeared in the exact order of their 

 minute imitations in the larval history of their descendants, and by 

 a comparative study of larval stages with adult forms, the naturalist 

 finds the key to relationships, and is enabled to arrange genera in 

 genetic series." 1 On this account, not less than for their inherent 

 attractiveness, .they merit foremost attention in the characterization 

 of the faunas. 



The earlier Triassic faunas. — A great group of ammonites, embracing 

 more than 200 species, formed the leading feature of the early Indian 

 Triassic assemblage of marine life. These ranged from the ceratite 

 family, whose sutures were alternately lobed and serrate (see Fig. 345, a), 

 to the true ammonites in which the sutures were as tortuous as the 

 outline of an arbor-vitas leaf. The Otoceras, with ear -like suture lobes 

 (vvhence the name), characterized the earliest stage, while Gyronites, 

 Proptychites , Ceratites, and Flemingites in succession characterized 

 the later stages. 



Among these later genera was the ceratite-like genus Meekoceras 

 (Fig. 345, c), which has special interest because it occurs also in western 

 and southeastern Idaho (Aspen Mountains) with the brachiopod genus 

 Terebratula (Fig. 345, h) and other forms that link together the Ameri- 

 can and Asian faunas. The alliance of these foims is sufficiently 

 close to indicate that before the close of the earlier Triassic epoch 

 migratory connections had been established between India and west- 

 ern America. It is significant in this connection that a fauna closely 

 related to this ceratite fauna of India occupied the Pacific border 

 m the vicinity of Vladivostok. In this are found a few species iden- 



1 James Perrin Smith, " Comparative Study of Palseontogeny and Phylogeny," 

 Jour, of Geol., Vol. V, 1897, p. 517. 



