414 
THE GEOLOGIST. 
impressions 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 i-esemblance to the last-named genus, the shell- 
structure of Spirifera does not present a vestige of that system 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 oi Spiriferina 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)3iV(/era-8haped species which 
possess a circular foramen, or aperture in the deltidium of the larger 
valvo — the author did not furnish us with any other information 
regarding the internal arrangements of his two named types, Cyrtia 
exporrecta and C. trapezoidalis. Subsequently to 1827, several other 
Bpccies 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 
to discover some of the characters of the following species : — C. exper- 
recta, C. trapezoidalis, C. Murchisoniana, C. cuspidata, C. heteroclyta, 
C. Demarlii, and C. sejptosa. The results of my examination will show 
that in the first four which belong to Dalman's genus, the internal 
characters are similar, but different from the last three, which cannot 
with propriety bo retained in the same group, or with the same 
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 
anim;il, and I have tlicrcfore proposed (at least provisionally) to dis- 
