LAMELLIBRANCHIATA. 255 



Mytilarca (Plethomytilus) oviformis. 



PLATE XXXI, FIGS. 1-8 ; AND PLATE LXXXYII, FIG S. 



I noceramus oviformis, Conrad. Jour. Acad. Nat. Sci., Phila., vol. viii. p. i'4(S. pi. 13, fig 7. 1842 

 Mytilarca oviformis (Conrad), Hall. Prelim. Notice Lam. Shells, etc., pt. 2. p. -2). 1869 

 Plethomytilus oviformis (Conrad). Hall. Pal. N. Y., vol. v. pt. 1. Plates and Explanations: PI. 31, 



figs. 1-8. Jan.. 1883. 



Shell large ; body ovate, erect, wide posteriorly and acute above ; length about 

 one-fifth greater than the height ; ventral margin for nearly half the length 

 of the shell nearly direct, thence gently curving into the posterior extremity 

 which is broadly rounded ; dorsal margin very gently curved. 



Valves equal, regularly convex in the posterior part, becoming gibbous 

 in the umbonal region. 



Hinge-line straight, less than the height of the shell. Beaks prominent, 

 anterior acute, and incurved, rising above the cardinal line. 



Ventral side truncate with a small fold for the passage of the byssus. 

 Dorsal side sub-alate. 



Test thick, marked by fine, close stria? of growth which at intervals are 

 fasciculate and raised into lamellose elevations and very much crowded and 

 elevated on the ventral side of the shell. 



Ligamental area wide, finely striated longitudinally and occasionally 

 preserving one or two oblique folds which may have been lateral teeth. 

 Other characters of the interior unknown. 



A large specimen has a length of 74 mm. and height 55 mm. A small 

 example measures 25 mm. in length and 17 mm. in height. 



This form differs from M. (P.) ponderosa in its greater proportional height, 

 smaller size, and lesser gibbosity. 



Formation and localities. In the shales of the Hamilton group at numerous 

 localities in Schoharie, Otsego, Onondaga. Ontario, Livingston ami Erie 

 counties, N. Y. 



