GENERA CYRTIA AND CYRTINA. 
CYRTIA of Dalman, and CYRTINA 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 exporrecta and C. trapezoidalis, both of which 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 
Dalman’s 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 the interal cavity, thus dividing 
it into two parts. 
