GENERA CYRTIA AND CYRTINA. 363 



CyrtiA of Dalman, and CyrTIIVA of Davidson. 



Following the general external characters of certain spiriferoid forms, as 

 described by Dalman in his definition of Cyrtia, those with semiconical 

 or semipyramidal ventral valves, high flat areas, with a narrow fissure 

 closed by a convex pseudo-deltidium, have been referred to that genus. 

 In some of these, there is a round perforation in the upper part of the 

 pseudo-deltidium ; but this feature may be present or absent, and has 

 not been regarded as of generic significance. The types of Dalman's genus 

 are the Cyrtia exporrecfa and C. trapezoidalis,' both, of Avhich are Silurian 

 species. Since the genus was proposed, several other European fossils 

 have been referred to it, and some American species have been added to 

 the list. 



As early as 1853, Mr. Davidson, in his General Introduction to the 

 Study of the Brachiopoda, had expressed some doubt as to the value of 

 DalmanIs Genus Cyrtia ; but it was not till 1858 that he proposed the 

 separation of certain species which had been referred to that genus, on 

 the ground of a want of correspondence in their parts with the typical 

 species of Cyrtia. These species, the Cyrtia heteroclita, C. demarli and C. 

 septosa, were found not only to possess the high area with closed fissure, 

 but also to show a peculiar modification of the dental plates of the ven- 

 tral valve, and the shell-structure is likewise punctate ; while in the 

 types of Dalman's genus the shell is impunctate, and the dental plates 

 are similar to those in Spirifer. 



In Cyrtina, as described and illustrated by Mr. Davidson, we have 

 small spiriferoid shells with the valves very unequal, the ventral valve 

 being extremely elevated and semipyramidal, with high area and narrow 

 fissure closed by a convex pseudo-deltidium, presenting the same aspect 

 with those heretofore referred to Cyrtia ; the dental plates converge 

 from the inner margins of the fissure, and uniting, form a single plate or 

 septum which reaches to the bottom of tire interal cavity, thus dividing 

 it into two parts. 



