914 PALEONTOLOGY OF NEW-YORK. 



volume has been passing through the press, a careful review and re- 

 examination of the specimens has enabled me to discover the existence 

 of internal spires, which, from the displacement of those first examined, 

 appeared to be arranged as in Atrypa; while farther examinations show 

 that these appendages are arranged as in Spirifer and Trematospira. 



The T. globosa and T. formosa have so much resemblance to Retzia in 

 general form and exterior markings, that they might readily be mistaken 

 for species of that genus ; but they have no area or extension of the 

 hinge line, which is almost always conspicuous on the dorsal valve of 

 Retzia. The shells of the latter are more finely striated and finely punc- 

 tate throughout, while we are able to distinguish only a granular or 

 punctate surface in Trematospira. 



In the United States, the Genus Retzia is yet unknown below the 

 Carboniferous limestone. 



Had the materials now possessed been originally studied altogether, 

 the whole, by some extension of the characters, might have been in- 

 cluded in the same designation ; but it is only since the description of 

 the genus has been printed, that I have obtained the means of showing 

 the similarity of structure. 



The following species, except T. deweyi, dilfer from those already described, in 

 being more extended like Rhynchondla^ the beak strongly incurved, and tlie lateral 

 slopes below the beak flattened or depressed as in that genus. 



The perforation of the beak, arrangement of the spire, and granular or apparent 

 punctate structure of the shell of the following species, correspond with the pre- 

 (Jeding species of Trematospira. The triangular depression beneath the beak, Avhich 

 in T. multistriata appears not to open into the cavity of the shell, is probably not a 

 real character; since in these species it is clearly an opening into the interior, the 

 beak of the dorsal valve being slightly inserted and held in place by a broad short 

 cardinal process which is clasped at base by the cardinal teeth of the opposite valve. 

 This intimate structure of the hinge has not been fully made out in the preceding 

 species of Trematospira; and should it hereafter be found identical, the two groups 

 may be united by some extension of the generic characters. 



From the general resemblance in external form and i)licated surface of these 

 shells to Rhynchonella, it is not improbable that some species referred to that genus 

 may prove identical with those now under consideration. 



For illustrations of the internal structure of those species described on Plate 

 zxxvi, and for T. rectirostra, see Plate xxxvi a. 



