102 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [nuu.87. 



CRURA AND CRURALIUM. 



Calcareous processes for tlie support of the bracliia are also devel- 

 oped in the l*rotremata, in the siiperfainily Peutameracea, but never 

 to the same degree attained by the Spiriferacea or Terebratulacea of 

 the Telotremata. In the Protremata these supports are first developed 

 in the Syntrophiida', and attain their greatest length in the Penta- 

 meridip. Since the two parts often unite medially, forming a plate for 

 muscular insertion either resting upon the valve or supported by a 

 septum, this has been termed a cruralium by Hall and Clarke, to distin- 

 guish it from the spondylium of the ventral valve. When the parts 

 remain separate, aud are therefore not for muscular insertion, they are 

 homologous with and the equivalent of the crura in the Ehynchonellid?e. 

 The crura of the Peutameracea aud JRhynchonellacea arise independ- 

 ently, aud are therefore morphologic equivalents. 



MORPHOLOGIC EQUIVALENTS. 



Because of the presence of similar or identical morphological struc- 

 tures iu different groups of mature brachiopods, it is unsafe, on the 

 basis of these alone, to suppose such to have close relationship. The 

 spondylium has been shown to originate independently in three orders: 

 Atremata, Protremata, and Telotremata. Identical mature loops have 

 resulted in different ways in two stocks of the same family, one boreal 

 (Dallin.ne) and the other austral (Magellauin.T). Flat and more or less 

 wide cardinal areas develop independently of one another in Protre- 

 mata and Telotremata (Spiriferacea). Cementation of valves takes 

 place at different and widely separated geologic epochs in Neotremata, 

 Protremata, and Telotremata, and shell plications arise from smooth 

 stocks in Pentameracea, Ehynchonellacea, Spiriferacea, and Terebra- 

 tulacea. Natural phylogenies can only be established upon ontogenies 

 checked by chronogenesis or geologic succession. 



SUMMARY. 



In North America there are 1,859 Paleozoic, 49 Mesozoic, and 11 

 Cenozoic species of fossil Brachiopoda. There are 110 species in the 

 Cambrian, 319 in the Ordovician, 311 in the Silurian, 663 in the Devo- 

 luan, and 478 in the Carboniferous. 



The remarkable scarcity of post-Paleozoic species in America is 

 supposed to be due not so much to the general decline of the class as 

 to great orograiihic movements during the close of the Paleozoic, which 

 produced complete barriers against the introduction of species from 

 other areas. 



Specific differentiation was most rapid in the Ordovician, having 

 exceeded the Cambrian representation more than three times. 



Thirty per cent of all American Paleozoic species had wide geo- 

 grapliic distribution, which is most j)ronounced in the Devonian and 



