102 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [nui.i..87. 



CRURA AND CRURALTITM. 



Calcareous processes for the support of the brachia are also devel- 

 oped in the Protremata, in the superfamily Pentameracea, but never 

 to tlie same decree attained by the Spiriferacea or Terebratulacea of 

 the Telotremata. In the Protremata these supports are first developed 

 in the Syutrophiidie, and attain their greatest lengtli in the Penta- 

 meridu'. Since the two parts often unite medially, forminf;- a phite 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, and are therefore not for muscular insertion, they are 

 homologous with and the equivalent of the crura in the Ehyiu'honellidie. 

 The crura of the Pentameracea and Khynchonellacea arise independ- 

 ently, and are therefore morphologic equivalents. 



MORPHOLOGIC EQUIVALENTS. 



Because of the presence of similar or identical morphological struc- 

 tures in 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 Ioojjs have 

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

 (Dallina') and the other austral (Magellanimp). 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, Rhynchonellacea, Spiriferacea, and Terebra- 

 tulacea. Natural phylogeuies can only be established u}»on ontogenies 

 checked by chronogenesis or geologic succession. 



SUMMARY. 



In North America there are 1,859 Paleozoic, 49 Mesozoic, and 14 

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

 Cambrian, 319 in the Ordovician, 311 in the Silurian, G63 in the Devo- 

 nian, and 478 in the Carboniferous. 



The remarkable scarcity of i)ost-Paleozoic species in America is 

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

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

 produced complete barriers against the inti-odnction of species from 

 other areas. 



Specific dilVerentiation was most rai)id in tlie Ordovician, having 

 exceeded the Cambrian representation more than three times. 



Tliirty i»er cent of all American Paleozoic species had wide geo- 

 grajthic distribution, which is most xironouuced in the Devonian aud 



