BRACHIOPODA. 53 



The accompanying figure of S. Walcotti, Sow- 

 erby, shows the great elevation of this wall, and 

 the broad scars of the adductor muscles upon its 

 lateral faces. This specimen indicates how im- 

 portant are the changes in the anatomy of the 

 animal, resulting from, or productive of this 

 median septum. The older species have furnished 

 no direct evidence of similar muscular attach- fig. 42. 



SpMferina fTaicoWi, Soworby; ahowingmus- 



ment, but there is no reason for doubting its cuiar scai son waiis of median septum of 



° podiclevalve. (c.) 



existence wherever such a septum is found. 



As far as observed, the loop of the palaeozoic species is slightly different from 

 that of the later members of the genus, and resembles that of Cyrtina, the 

 lateral portions converging upward, between the spiral coils, and uniting in a 

 slight anterior extension. The spiral ribbon is spiniferous in S. rostrata, but 

 usually smooth in the Carboniferous species. In S. spinosa, and probably in 

 other species, there is a solid calcareous deposition in the umbonal cavity of 

 the pedicle-valve, filling the interspaces between the dental lamellae and the 

 median septum, not constituting a union of the three plates as in Cyrtina, but 

 forming a secretion analogous to that found in the syringothyroid Spirifers, 

 and to the transverse plate in Syringothyris itself Both the palaeozoic and 

 Liassic species have broad crura, a faint elevated median ridge in the brachial 

 valve, and a pair of divergent ridges lying on the surface of the first 

 internal plications, extending fully, or more than one-half the length of the 

 valve, and ending abruptly; probably the external fulcra of the adductor 

 muscles. 



It has already been observed that the derivation of the generic characters of 

 this genus has been from the lamellose-septate Spirifers whose inception dates 

 from the faunas of the Upper Silurian. Though none of these Silurian and 

 Devonian species, in the American faunas, developed a punctate shell structure, 

 they usually bear the lamellose, often radially striated exterior, prevailing 

 among the Spiriferinas of the Carboniferous. Mr. Davidson has described two 

 of these lamellose species from the Devonian, which have a strongly punctated 



