LAMELLIBRANCHIATA. 277 



MODIOMORPHA MYTILOIDES. 



PLATE XXXVII, FIG. 3; and PLATE XXXVIII, FIGS. 1-16. 



CypHcardites mytihidfs, Conrad. Geol. Smv. N. Y., Ann. Rep., p. 52. 1S41. 



Modiomoryha complanata. Hall. In error. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 38, 



fijfs. 1-16. 188:^. 

 Modi-iaorpJia plannlata, " Pi-elim. Notice Laniellibranchiata, 2, p. 74. 1870. 



Shell larger than the medium size, rhomboid-ovate, oblique; length less than 

 twice the height; basal margin nearly straight, or very slightly concave 

 anterior to the middle, curving to the anterior and ;posterior extremities; 

 posterior margin abruptly curving below and more gently recurving toward 

 the cardinal line ; cardinal margin arcuate. Anterior end narrow, extended, 

 abruptly curved on the margin; somewhat defined by the sinus which extends 

 from anterior to the beak to the middle of the shell. 



Valves moderately convex ; in old shells gibbous in the umbonal region. 



Hinge-line oblique, extending to about the middle of the length of the shell. 



Beaks appressed, situated a little more than one-fourth the length of the 

 shell from the anterior end. Umbonal region not defined ; convex in young 

 shells, becoming more gibbous in older individuals. 



Test of moderate thickness, concentrically striated with irregular lines of 

 growth which are sometimes elevated into concentric ridges. The post- 

 cardinal slope in well-preserved specimens shows fine vascular markings 

 similar to those on Sphenotus soknoides. 



The anterior muscular impression is well marked and situated just within 

 the anterior margin below the beak. Other interior characters unknown. 



The specimen fig. 13 of plate xxxviii preserves the ligament connecting 

 the two valves. 



Three characteristic specimens have the following dimensions : Length 50, 

 60, 91 mm. ; height 29, 37 and 50 mm. respectively. 



The individuals of this species from medium size upward resemble M. con- 



cenlrira in form, but the anterior end is more produced and they do not show 



the regular sharp concentric striiB which mark that species. It also resembles 



some forms which are referred to M. alia, but may be distinguished by its more 



36 



