NUCL.EOSPIRA 457 



Surface of both valves marked by fine, concentric lines of growth. 



Remarks. This species is much smaller tha.n N. barrisi and does not pos- 

 sess the rather deep mesial sinus which is commonly developed towards 

 the anterior margin in that species. It is larger and more nearly circular 

 than A T . rowleyi. 



Horizon. Glen Park limestone of the Kinderhook. 



NUCLEOSPIRA OBESA Rowley 



Plate LXXXII, Figs. 39-42 



1900. Nucleospira cbesa Rowley, Am. Geol., vol. 25, p. 263, pi. 5, figs. 

 38-40. 



Description. Shell small, lenticular, subcircular in outline, usually 

 slightly longer than wide, the anterior margin slightly truncated, the 

 greatest width near the mid-length of the shell, the hinge-line much 

 shorter than the greatest width, the cardinal extremities rounded. The 

 dimensions of a very perfect specimen, a metatype, are : length of pedicle 

 valve 6.3 mm., length of brachial valve 5.9 mm., greatest width 6 mm., 

 thickness 4 mm., length of hinge-line 2.6 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly from the umbonal region to the cardinal margin on each side of 

 the beak, curving more gently to the lateral and anterior margins; mesial 

 sinus obscure, originating at or near the beak as a slight flattening of the 

 valve, becoming slightly depressed in the umbonal region and continuing 

 to the anterior margin as a narrow and shallow depression with very 

 gradual increase in width and nowhere sharply defined; beak short and 

 small, sharply pointed and moderately incurved; cardinal area small, 

 concave, sloping posteriorly at an angle of about 45 degrees to the plane 

 of the valve, the lateral margins well defined. Internal characters not 

 observed. 



Brachial valve nearly equally convex with the pedicle, the greatest con- 

 vexity posterior to the middle, the surface curving somewhat more ab- 

 ruptly to the posterior than to the lateral and anterior margins, the um- 

 bonal region rather prominent and slightly flattened along the median 

 line; near the middle of the valve a slight median sinus originates and 

 continues to the front margin, this sinus being even fainter than that of 

 the opposite valve ; the beak rather strongly incurved beneath the delthy- 

 rium of the opposite valve. Internal characters not observed. 



Surface of both valves, in the specimens observed, nearly smooth, 

 marked only by obscure concentric lines of growth. 



Remarks. The fine appressed surface spines which are commonly pres- 

 ent on members of this genus have not been observed in this species, prob- 

 ably because of the condition of preservation. The species differs from 

 N. barrisi in its smaller size, more nearly circular shell, and in the absence 



