964 PALEONTOLOGY OP NEW-YORK. 



This arrangement leaves an enclosed triangular tube between the 

 pseudo-deltidiura and the converging dental plates. To these characters 

 may be added the punctate texture of the shell, which, so far as now 

 known, is accompanied in these forms by the external character and 

 internal arrangements described. 



Besides these features, several of > our species have a median septum 

 in the fissure, in continuation of the central septum below the 

 junction of the dental plates. This septum is visible when the pseudo- 

 deltidium is removed, as shown in figures of C. rostrala*, and as seen in 

 many specimens of C. dalmani, as well as in C. biplicata and in C. hamil- 

 ionensis. This central septum, both above and below the junction of the 

 dental plates, may have the same origin : the coalescing, of these 

 plates allows the exterior lamina? of each to unite, and extending inward 

 form the septum dividing the cavity ; while the inner walls of the dental 

 plates are united and recurved, turning outwards to form the septum 

 dividing the triangular space beneath the pseudo-deltidium. . 



The several species Avhich I have heretofore described as Cyrtia have 

 all a punctate structure, and, so far as examined, they have the arrange- 

 ment of the internal parts described in Cyrtina. It was not until after 

 the completion of the third volume of the Palaeontology of New-York 

 in 1859, that I received that part of Mr. Davidson's Monograph of the 

 Carboniferous fossils containing his arguments for the separation of the 

 genera and his description of Cyrtina. The characters of Cyrtina, as 

 given by Mr. Davidson, show its near relations with Spiriferina ; the 

 principal diiferences, as illustrated in figures of the latter genus, being 

 that the dental plates do not coalesce before reaching the bottom of the 

 cavity, the high median septum rising unsupported nearly to the plane 

 of the area. 



In that volume I have described twb species under the name of Cyrtia, 

 both of which have the structure of Cyrtina ; and in one of these I men- 



tioned the discovery of the spiresf, a feature which Mr. Davidson had not 



_ . 



• PalsBontolopy of New- York, Vol. iii, pi. 9C, f. 2 rf, 3 a & 4 d. 

 t Cyrtia rostrala, Pal. New- York, Vol. iii, p. 429. 



