286 PALAEONTOLOGY OF NEW YORK. 



MODIOMORPHA RECTA, n. Sp. 



PLATE X«XV, FIG. 9. - ' 



Shell of medium size, sub-ovate ; length less than twice the height ; basal 

 margin nearly straight, or gently convex, curving abruptly at the anterior 

 and posterior extremities. Posterior end somewhat abruptly rounded. Car- 

 dinal margin nearly straight. Anterior end short, abruptly rounded at the 

 extremity ; indistinctly limited by a broad flattening of the valves extending 

 from the umbo to the base of the shell, about one-third the length from the 

 anterior end. 



Valves very convex, gibbous on the middle and umbonal regions, somewhat 

 abruptly declining toward the anterior and posterior extremities; depth less 

 than one-half the length of the shell. 



Hinge-line straight, slightly oblique, a little less than half the length of 

 the shell. 



Beaks sub-anterior, small, closely appressed. Umbonal region gibbous to 

 about the posterior third, thence regularly declining to the margin. 



Test of moderate thickness, concentrically striated. 



Anterior muscular impression strong, with an accessory muscular pit. Pal- 

 lial line not deeply impressed, following the general course of the basal 

 margin, recurving at the posterior extremity, and terminating in a broad 

 posterior muscular impression. The space between the pallial line and um- 

 bonal region is marked by pitted vascular lines. 



One of the specimens described has a length of 57 mm., height 32 mm., 

 and depth of both valves 25 mm. Another specimen measures 52 mm. 

 in length and 30 mm. in height. 



The general form of the shell is similar to M. subalata, but it has not the 

 distinct umbonal ridge, nor alate post-cardinal slope. 



Formations and localities. In the cherty-layers of the Hamilton group, in 

 Clarke county, Ind. ; and in the Corniferous limestone of Canada West. 



