BRACHIOPODA. 41 



the original diagnosis no consideration was given to internal characters, and 

 Davidson subsequently demonstrated that these semipyramidal Spirifers repre- 

 sent at least two different types of interior. He therefore restricted Cyrtia to 

 Dalman's first two examples, C. exporreda, Wahlenberg, and C. irapezoidalis, 

 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 semipyramidal 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 tliese 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 exporreda itself,* and is a more constant character 

 in the larger Devonian species C. Murchisoniana, 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 lamellae, 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 Brachiopoda, p. 68. 



