100 Trans. Acad. Sci. of St. Louis. 



It seems to be distinct, however, from the specimen specific- 

 ally designated as the type specimen, and furthermore, seems 

 to be the more common form of the two. It differs from 

 /S. iowensis in the absence of the sharply angular or carinate 

 umbonal ridge, and also in the additional radiating ridge upon 

 the postero-clorsal slope. The lines of growth are also less 

 conspicuous than in S. ioivensis. 



Spathella ventricosa (W. & W.) 



PI. IV. f. 12. 



The types of this species are two in number, the larger one 

 being 45 mm. in length, and 20 mm. in height ; the smaller 

 one is 21 mm. long and 10 mm. high. The shell is subcylin- 

 drical, widest behind. Hinge-line straight, about two-thirds 

 the length of the shell; anterior end short, the margin ab- 

 ruptly rounded; ventral margin nearly straight or slightly 

 arcuate throughout the greater part of its length, curving 

 upward into both extremities ; posterior margin usually regu- 

 larly rounded, forming a segment of a circle, sometimes more 

 abruptly rounded. Beak anterior, not prominent, small and 

 closely incurved. Valves very convex, gibbous in the middle ; 

 umbonal ridge broad and ill-defined, the dorsal slope much 

 more abrupt than the ventral. Surface marked by fine con- 

 centric lines of growth, which are sometimes fasciculate in the 

 posterior portion of the shell. 



Remarks. Of the two type specimens of this species the 

 smaller one is illustrated, the larger one being imperfect in 

 the anterior portion. Upon the specimens illustrated by Hall, 

 the concentric growth lines were evidently much more irregu- 

 lar and fasciculate than in the original types. 



Cardiopsis megambonata Win. 



PI. III. f. 18. 



Shell subcircular in outline, strongly ventricose, oblique ; 

 length from the beak to the postero-basal margin 25 

 mm., height from ventral to dorsal margin, and length 

 from anterior to posterior margins, each 20 mm., con- 

 vexity of valve, 10 mm. Hinge-line short, pallial margin 

 regularly rounded. Beak prominent, situated anteriorly, 



