60 PROCEEDINGS OF THE ACADEMY OF 



LAMELLIBRANCHIATA. 



AVICULOPECTEN CKENISTEIATUS, Meek. 



Shell thin, attaining a moderatel}'^ large size, plano-convex, "oith 

 a truncato-suhoi'bicular outline ; height and breadth nearly equal : 

 basal margin more or less regularly rounded ; posterior margin 

 rounding from the posterior ear into the base, sometimes a little 

 straightened above ; anterior side rounded so as to make a shorter 

 curve than the other ; hinge less than the breadth of the valves 

 in length, but rather long. 



Left valve moderately convex ; beak rising a little above the 

 hinge margin, nearly or quite central and not oblique ; posterior 

 ear small, with a marginal ridge, flat, without being separated 

 from the swell of the umbo by a sulcus, rather acutely angular 

 at the extremity, and distinctly shorter than the margin below, 

 from which it is separated b}' a nearly rectangular notch ; ante- 

 rior ear larger, or sometimes nearly as long as the margin below, 

 rather acutely angular at the extremity, convex and separated by 

 a rounded impression from the umbo, defined by a rather deep 

 subangular marginal sinus. Surface ornamented by numerous, 

 very slender, radiating costre or raised lines, separated by rather 

 wide flat surfaces, in each of which a still smaller line is some- 

 times intercalated ; crossing all of these are smaller regularly 

 arranged, sharply elevated, concentric lines, that seem to form 

 little projections at the points of crossing, so as to give a rough 

 appearance to the surface ; costae becoming more closely crowded, 

 but not smaller, on the ears. 



Right valve flat or a little concave, and with beak obsolete ; 

 ears flat, and of nearly the same size and forifl as in the other 

 valve, excepting that the anterior one is broader and defined b3' a 

 more shallow and more rounded sinus than appears to exist in 

 the left valve ; surface cancellated by regular, rather closely ar- 

 ranged, radiating and concentric lines, the latter being very 

 sharply elevated, and minutely and regularly waA^ed, crenate, or 

 somewhat vaulted ; radiating costai somewhat larger, and more 

 distant on the ears. 



Height, 2.50 inches; breadth, 2.78 inches; convexitj^ about 

 0.40 inch. 



The specimens of this species are not very well preserved, being 



[June 6, 



