o - 



Diniensions .- Length, 31 nira. ; alt., 16.7 vAm. ; semidiam., 7 

 mm.; length of hinge, 19 ram.; length, 28 mm.; alt., 15.8 mm.; 

 semidiara. , 6.5 mjn. ; length of hinge, 11 mm. 



5emar':s .- Jompared with the average Scapharea the shell 

 of iiiis species is decidedly tb in and the sculpturing is elegaiit. 

 Individuals belonging to this snecies are numerous and they may he 

 inraediately recognized by their small size, thin shells and inter- 

 nal radiating lints. -"he differences in the size and sculpturing 

 of the radial ribs in different areas of the shell are enhanced 

 by the delicate chiseling vihich characterizes the sculpture through- 

 out, '-^e anterior ribs are nodulated and sulcated; the me-'ian 

 ribs are na-row and usually not prominently nociulate''; the poster-, 

 ior ribs are broad, low and smooth, exGe"ot for a shallow groove. 

 A few forms shov/ .a very faint and shallow sulcus on the umbo. Some 

 young shells have the dorsal postero-lateral nargin more deenly in- 

 cised than in the case of the average adult. A. inaeguilateralis 

 is closely related, as Dall (1) has pointed out, to A. latidentata 



(l). loc. cit. 



Dall (2) of the Ohipola marl and Tampa siles be is, but nay be dis- 



iP-), Ice. cit., p. 638, pi. r-g, fig. 15, 1898. 



