«7'J PALAEONTOLOGY OF NEW-YORK. 



hirsuta, has been found to have a punctate structure, with spires arran 

 ged as in Spirifera, and otherwise possessing the characters of Trema- 

 TOSPIRA ; and another species from the group has been added to the 

 number. 



Desirous of avoiding too great a multiplication of genera, I at first 

 included under this designation species which it has since been found 

 necessary to separate on account of marked difference in form, and also 

 in the character of the hinge appendages. These latter species, under the 

 name of Ruynchospira, seem to me sufficiently distinct from Retzia to 

 be recognized at least as a subgenus. 



In the present state of our knowledge of the heterogeneous assemblage 

 of material known under the name Retzia, it is quite impossible to 

 define the limits of that genus with any precision. Farther investiga- 

 tions, with better material, are required for this purpose ; for the deter- 

 mination by external form alone is not satisfactory. In the mean time, I 

 refer tlie following forms to the genera under which they are placed 

 according to the best information we have been able to obtain of their 

 structure and affinities. 



Treuiatosiiiia gibbosa. 



PLATE XLV. 



Trematoepira gibbota : Hail, in Thirteenth Report on the State Cabinet, p. 82. I8C0. 

 Retzia eugenia [?] Billisgs, Canadian Journal, p. 147. 1861. 



Shell more or less gibbous : valves subequally convex, transversely 

 subelliptical, about once and a half as wide. as long; hinge-line less 

 than the width of the shell ; strongly plicate. 



Ventral valve less gibbous than the opposite, somewhat abruptly 

 arcuate towards the apex, which is truncated by a circular foramen : 

 this is completed on the inner side by what appear to be depressed 

 deltidial plates, which at the same time form a flattened triangular 

 space or false area. Mesial sinus abruptly depressed in old shells, and 

 less conspicuouly in younger ones. 



