LAMELLIBRANCHIATA. 279 



Beaks rounded, somewhat appressed, directed forward. Umbonal region 

 not strongly defined, depressed anteriorly, becoming gibbous in the middle 

 of the shell, gradually merging into tlie general contour in the posterior 

 portion. 



Test of moderate thickness, marked by irregular concentric striae, which 

 become fasciculate and produce strong concentric ridges at irregular 

 intervals. The surface is marked by fine vascular lines, similar to those 

 referred to in M. mytiloides. The anterior muscular impression is situated 

 close to the anterior margin of the shell, with a small retractor scar above it. 

 Other characters of the interior unknown. 



The type specimen has a length of 68 mm., and a height of 46 mm. 

 Three other specimens measure respectively, 65, 55 and 67 mm. in length, 

 and 45, 37 and 42 mm. in height. 



This species has a greater proportional height than M. mytiloides, a broader 

 and often sub-truncate posterior end, while the anterior end is usually broader 

 and less extended ; the umbonal ridge is arcuate ; in other features it is very 

 similar. The form does not attain so large a size as M. macilenta ; it is more 

 ventricose in the umbonal region, the hinge-line is longer and the striae are 

 more irregular. In some of its conditions this species approaches M. mytiloides, 

 and it is not always easy to distinguish them. 



This species shows a tendency to assume extremely gibbous and sometimes 

 distorted forms, as shown in fig. 2 of plate xxxvii and fig. 7 of plate Ixxx. 

 The specimen fig. 15 of plate xxxvii, is the type of this species. 



Formations and localities. In the shales of the middle portions of the Hamilton 

 group at many localities in the eastern and central part of the State, and 

 especially on the shores of Seneca and Cayuga lakes ; in the Hamilton group 

 at the Falls of the Ohio, and Charleston, Indiana. Small individuals having 

 the characteristics of this species occur in the Marcellus shales near Skanea- 

 teles, N. Y. 



