MYTILACEA. MOLLUSCA. MODIOLA. 127 



This species seems to be subject to little variety. Sometimes, 

 however, we find the lower margin nearly straight. In this case the 

 upper margin is nearly parallel to the lower, so that the shell increases 

 but little in height ; and in such shells the beaks appear much more 

 prominent than in ordinary specimens. 



The principal variations seem to consist in the coloring, which is 

 made up of various shades from bright-yellow, passing through horn- 

 color and chestnut to dark bronze-green, and arranged in various 

 modes and in various proportions. What Mr. Conrad regards as a 

 distinct species under the name semi-costdta, certainly can be nothing 

 more than a variety of this shell. 



Modi' OLA pectinula. 



Shell obovate^ ventricose^ ivith about forty equal, radiating 

 ribs ; beaks prominent, projecting as far as the anterior margin ; 

 epidermis brownish yellow. 



Figure 85. 



State Coll., No. 159. Soc. Cab., No. 2342. 



Shell small, strong, of a strictly ovate form, excepting that a 

 very short portion of the hinge margin is straight ; broadly round- 

 ed at both extremities, but most so behind ; valves convex, with- 

 out any marked ridge passing from the beaks, and very little com- 

 pression at the hinge margin ; beaks rather prominent, blunt, and 

 in contact with each other, reaching forward as far as the anterior 

 extremity ; surface covered with about forty equal, rounded, 

 radiating ribs ; epidermis a dark gamboge-yellow ; lines of growth 

 minute ; within pearly, of a livid or leaden color ; entire 

 margin crenulated by the ribs ; cavity of the beaks considerable. 

 Length x^^ inch, height | inch, breadth ^jj inch. 



Inhabits St. George's Bank. 



This very strongly marked species seems to have been hitherto 

 undescribed. It is closely allied to M. discrepans, but is smaller, 

 stronger, lighter-colored, and entirely covered with ribs. Of five or 

 six specimens which I have seen, all apparently mature shells, the 

 largest was of the size given above. M. drctica, Leach, is a smaller 

 and more rounded shell, and the ribs are much more numerous. 



