PELECYPODS 



425 



A. aculeata is much smaller than the last species, 

 and the surface of the convex valve is roughened 

 by prickly scales. The smaller valve is very thin, 

 with an almost circular aperture for the byssus. 

 Its diameter is about half an inch. Found at- . JnomiaacMtota ,fro m ,,. 



tached to Stones, etc., about the holdfasts Of FuCUS. AnomiaacuUata, frombelow. 



A. lampe, a California species ranging more 



to the south, is small (half an inch), and greatly resembles the east- 

 coast A. aculeata, save as regards the smooth surface of the former. 



GENUS Placunanomia 



P. macrochistna. This species occurs on the Calif ornian coast. It has 

 the same sort of calcined byssus as Anomia, and also the hole to accom- 

 modate it in the smaller valve. The scar formed by the muscle which 

 controls the byssus is curiously rayed in this genus. The normal shape 

 of the shell is circular ; it is pearly within, and of a greenish tinge with- 

 out. Two to four inches in diameter. (Plate LXXVIII.) 



FAMILY AECIDJE 

 GENUS Area 



The Arcidce may always be distinguished by the long row of 

 comb-like teeth upon the hinge, the total want of siphons (a fact 

 which may be discovered in the shell alone by its lack of a pallial 

 sinus), and the solid trapezoidal or round shell, with its tendency to 

 strong ribs radiating from the umbones. The periostracum, or 

 epidermis, is heavy and often velvety or even hairy. The animal 

 of Area has the gills placed in an oblique position ; the foot is 

 large and strongly developed. No trace of mantle fusion exists. 

 The mantle-edges bear a row of composite eyes. The gill-fila- 

 ments are entirely free, there being even no ciliary junctions. 



A. pexata. An exceedingly 

 common species in Long Island 

 Sound and on the coast of New 

 Jersey. The shell is oblong, with 

 prominent beaks directed forward. 

 The hinge-teeth are arranged in 

 I a row posterior to the beaks, while 

 W just under the beaks are a num- 

 ber of irregular cartilage-pits. 

 About tlm-ty-two to thirty-six ra- 

 diating ribs ornament the shell. 

 The inner margins of the shell are 

 rather deeply scalloped; the epi- 

 dermis is thick, shaggy, and dark 



