272 
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 Riiynchospira, seem to me sufficiently distinct from Retzia to 
he 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 the 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. 
Trematospira gibbosa. 
PLATE XLY. 
Trematospira gibbosa : Hall, in Thirteenth Report on the State Cabinet, p. 82. 1860. 
Retzia eugenia [ ?] Billings, 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. 
Yentral 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. 
