16 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [hull. 87. 



Table ahouing the differentiation of the Brachiopoda during Cambrian time. 



The earliest deep-water deposits of tlie Silurian, the Clinton forma 

 tion, have a brachiopod fauna which is quite different from that of the, 

 Ordovician. The Atremata, Neotremata, and Protremata are much 

 like those of the Ordovician, but the Spiriferacea of the Telotreuiata, 

 the most characteristic brachiopods of the Silurian, have here attained 

 a great variety of forms, with varied brachydial structures. Through- 

 out the American Silurian the brachiopods show little structural differ- 

 entiation, but in the Lower Helderberg, at the base of the Devonian, 

 the spire-bearers are changing and assuming characters which are fully 

 developed in the higher Devonian. Here also occur the oldest loop- 

 bearers, or Terebratulacea, though the ontogeny of Zygospira seems to 

 show that this superfamily originated in the Ordovician. 



In the Mississippian Sea deposition was apparently quite continuous 

 throughout Devonian and Carboniferous times, and not much inter- 

 ru})ted by earth movements. The faunas of these systems in this area 

 show no rapid evolution along any of the brachiopod phyla. The 

 species of the basal member of the Carboniferous, the Waverly or 

 Kinderhook, are not unlike those of the Chemung of the Upper 

 Devonian, nor is there any great faunal difference between the Kas- 

 kaskia of the Lower Carboniferons and the productive Coal Measures 

 above. 



From the foregoing rapid summary of the geologic history of Amer- 

 ican brachiopods, it follows that differentiation in the Paleozoic is most 

 rai)id near the base of the older systems, and diminishes in force from 

 the older to the younger geologic divisions. While earth movements 

 in America were greater and more numerous during the early Paleozoic 

 than later in and just previous to the close of this time, yet the early 

 and rapid evolution of the class is probably due not only to the varying 

 conditions produced by these movements but also to the greater plas- 

 ticity of the class during the Cambrian and Ordovician eras. 



There are 311 species in the American Silurian, increasing to 662 in 

 the Devonian, while the Carboniferous representation declines to 478 

 species. In 1880 Zittel gave a total of 1,366 species for the Devonian, 

 871 for the Carboniferous, and but 30 for the Permian. Waagen's 

 researches in the Permian of India, however, have increased this 

 representation considerably. 



There is no more striking evidence than these figures needed to show 



