414 



THE GEOLOGIST, 



iraprefsions in the interior of the shell as we perceive in the corres- 

 ponding valves of Terebratula. 



The sub-genus Spiriferina presents some well-marked features by 

 which it may be distinguished from Spirifera proper. Thus, notwith- 

 standing its external resemblance to the last-named genus, the shell- 

 structure of Spirifera does not present a vestige of that ^ystem of canals 

 or perforations, which traverse the shell of all the species of Spiriferina 

 hitherto discovered, and which peculiarity must have been accompanied 

 by other important modifications of the animal structure which cannot 

 be entirely ascertained ; but certain evidence of this is evinced by a 

 difference in the interior arrangement of the larger or ventral valve,— 

 for in all species Sjnriferina there exists, between the diverging dental 

 or rostral plates, a largely-developed mesial septum which is never present 

 in Spirifera, and to which was attached the adductor or occlusor muscle. 

 Spiriferina, therefore, although more nearly allied to Sjnrifera, and 

 perhaps to Suessia, than to any of the other genera, may advantage- 

 ously preserve the name, to designate those species which, in addition 

 to a perforated shell-structure, present a largely-developed mesial 

 plate in the interior of the ventral valve. The sub-genus Cyrtia was 

 created by Dalman to receive those few >S^ir//€m-shaped species which 

 possess a circular foramen, or aperture in the deltidium of the larger 

 valve — the author did not furnish us with any other information 

 regarding the internal arrangements of his two named types, Cyrtia 

 cxporrecta and C. trapezoidalis. Subsequently to 1827, several other 

 ppecies were added (by different authors) to the so-termed genus Cyrtia, 

 and among these are some the shell-structure of which has been stated 

 to be perforated, while that of Dalman's type is unpunctate, as in 

 Spirifera proper ; but it was not until very recently that I was enabled i 

 to discover some of the characters of the following species : — 0. exjy&r^ ! 

 i'ecta, C. trapezoidalis, C. Murchisoniana, C. cuspidata, C. heteroclyta, j 

 C. Demarliiy and C. septosa. The results of my examination will show j 

 that in the first four which belong to Dalman's genus, the internal j 

 characters are similar, but different from the last three, which cannot ! 

 with propriety be retained in the same group, or with the same j 

 denomination, for it is evident that considerable dissimilarities in the | 

 arrangements of the plates of the ventral valve must have carried ; 

 along with them some important differences in the soft portions of the I ; 

 nnimnl, and I have therefore proposed (at least provisionally) to dis- 



