88 Trans. Acad. Sci. of St. Louis. 



tion in their condition of preservation. Judging from Win- 

 chell's measurements it is the larger one of these two which 

 is really the type of the species. Most of the specimens are 

 smaller than this one, some of them having a length no greater 

 than 15 mm. 



The species approaches more closely to P. profundus Hall, 

 from the Chemung fauna of New York, than any other species 

 of the genus, but the two are quite distinct. P. whitei is a 

 much narrower shell and lacks the pointed anterior ear of 

 P. profundus. 



Mytilarca occidentalis (W. & W.). 



PI. III. f. 11. 



Shell elongate, narrowly ovate; length, 52 mm., breadth, 

 20 mm. Hinge-line short; ventral margin gently curving 

 from just below the beaks to the abruptly rounded posterior 

 margin; dorsal margin gently curving to the posterior ex- 

 tremity of the short hinge-line. Beaks acute, situated at the 

 extreme anterior end of the shell ; valves convex in the pos- 

 terior part, becoming gibbous anteriorly, the greatest con- 

 vexity anterior to the middle; the umbonal region narrow 

 and the convexity continued along the median line to the 

 posterior extremity of the shell. Surface marked by fine, 

 more or less irregular concentric striae, and at irregular inter- 

 vals by stronger concentric wrinkles. 



Remarks. The specimen here illustrated is the type of the 

 species. It is somewhat compressed dorso-ventrally, so that 

 it is proportionately narrower than before this distortion took 

 place, and the convexity along the median line is proportion- 

 ately accentuated. Hall's* illustration of the species was 

 drawn from the same specimen, but that figure is made to 

 represent the shell as restored to its normal form. 



Mytilarca fibristriata (W. & W.). 



PL III. f. 12. 



Shell elongate, ovate, very oblique; length, 41 mm., 

 breadth, 16 mm. Hinge-line short ; ventral margin nearly 

 straight in the central portion, curving gently upward to the 



* Pal. N. Y. &\pl. 87. f. 11. 



