38 INVERTEBRATA OF MASSACHUSETTS. 



anterior ; somewhat acutely rounded before, obliquely truncated 

 and widely gaping behind, the posterior margin thickened within 

 and turning outwards ; the hinge and basal margins are usually 

 about parallel, but in old shells the lower and posterior angle is 

 considerably prolonged, so as to render this portion broadest ; and 

 it is the only point at which the valves meet, the rest of the base 

 widely gaping ; beaks rather prominent, directed slightly forwards, 

 and from them extend two bioad, wave-like ridges, one directed 

 to the lower posterior angle, the other dividing the portion anterior 

 to this into two nearly equal parts, so that the surface is thus di- 

 vided into three triangular, concave compartments ; surface also 

 ridged at the stages of growth. Directly under the beak in each 

 valve is a single small, triangular tooth ; these shut side by side ; 

 running backwards from each of them, along the margin, is a thick, 

 rounded, crest-like callus, having a groove at its external base in 

 which a strong ligament is fixed, which arches over these crests. 

 Muscular impressions deep ; palleal impressions looking like an 

 irregular series of muscular pits of various sizes ; interior smooth 

 and shining, corresponding to the external undulations ; exterior 

 antiquated, livid. Length 2| inches, height I4 inch, breadth 

 lyo to lj% inch. 



Inhabits the Banks of Newfoundland, whence it is brought 

 by fishermen. 



I believe this to be the shell which Lamarck intended by his Ghjcy'm- 

 eris urctica, and which Deshayes, with good reason, pronounces to be 

 a Panop^'a. I am aware that the P. AJdrovdndi varies much at dif- 

 ferent ages, and has consequently been described under several names. 

 It is also said to be an inhabitant of Newfoundland, while Lamarck 

 gives the "Arctic Ocean, the White Sea," as the habitat of P. arc- 

 tica. But P. Aldrovdndi never presents upon the disc the two ridges 

 and intervening central valley, so characteristic of our shell ; it is also 

 nearly equilateral, broadest before, and the anterior extremity is scarce- 

 ly more rounded than the posterior, and even at the immense size, to 

 which that species often arrives, it is scarcely more thickened than our 

 small shell, which, indeed, bears evidence of entire maturity. The 

 " costis duabus obtusis," and the remark, that " externally it resembles 

 Mya truncdta,'''' are enough to identify the shell. A single valve would 

 be passed over as the tooihless valve of Mya truncdta. 



