BRACHIOPODA. 
41 
the original diagnosis no consideration Avas given to internal characters, and 
Davidson subsequently demonstrated that these seinipyramidal Spirifers repre¬ 
sent at least two different types of interior. He therefore restricted Cyetia to 
Dalman’s first two examples, C. exporrecta, Wahlenberg, and C. trapezoidalis, 
Dalman; considering the latter as but a variety of the former, and separated 
from this association shells of the type of the Calceola heteroclita, Defrance, 
which have a punctate shell structure and the dental plates conjoined with a 
median septum. To the latter he applied the term Cyrtina, and in so doing 
by far the larger number of the seinipyramidal spiriferoids were removed from 
Dalman’s genus. Cyrtia now stands as the designation of a group with a very 
meager representation and of very slight morphological value. 
The general habit of these shells is the coexistence of the vertical cardinal 
area with a convex deltidium perforated by a circular, oblique foramen ; at the 
same time the cardinal area may be incurved to a considerable degree, as is 
apparent in the species C. exporrecta itself,* and is a more constant character 
in the larger Devonian species C. Murchisoniam, de Koninck. 
At the time Davidson established the genus Cyrtina, he expressed the 
opinion that Cyrtia “ presents no other feature by which it can be separated 
from Spirifer proper, than that of its deltidium and foramen, which are charac¬ 
ters of hardly sufficient importance to warrant the creation of a separate 
genus.”f Though more than thirty years have elapsed since this judgment 
was expressed, it is fully supported by the evidence of to-day. Neither in the 
development of the dental lamellge, the form of the brachial attachments, nor 
in the muscular impressions can be found any other basis for distinction from 
Spirifer than that indicated, namely, the structure of the deltidium; and it is 
quite clear that in both Spirifer, Cyrtia and Cyrtina, this character has had 
the same mode of development. 
Notwithstanding these considerations which demonstrate the inferior generic 
value of this term, there is a certain external expression in these fossils, both 
in contour and ornamentation, which will not permit their association with 
* See Davidson’s figures given upon plate ix of his Silurian Brachiopoda. 
t British Carboniferous Brachiojioda, p. 68.* 
