174 ARCID&. 



he referred to the latter species as " A. raridentata, var. 

 major" Having carefully inspected and compared a 

 great nmnber of specimens, recent and fossil, reputed 

 to belong to both species, and having had the advantage 

 of examining on the spot the grounds upon which Sars 

 formed his opinion, I am not satisfied that these species 

 ought to be united. At first sight, indeed, it might 

 seem as if A. glacialis were only a large form of A. 

 pectunculoides, and that the difference of size was ex- 

 plained by the former having a more northern habitat. 

 But since A. pectunculoides has been found in Green- 

 land, and A. glacialis is not uncommon in Iceland, while 

 each constantly exhibits its own distinctive features, and 

 attains nearly an equal size in the Arctic seas, we must 

 inquire whether there is any intermediate link or variety 

 connecting the two forms. I believe this question must 

 be answered in the negative, so far as our present know- 

 ledge extends. The shell of A. pectunculoides is round- 

 ish-oval or trapezoidal ; that of A. glacialis is obliquely 

 oblong. The former is more gibbous or convex than 

 the latter ; the length, or distance from the umbo to the 

 front margin, is proportionally greater ; the beaks are 

 more prominent and placed nearer the centre of the 

 hinge-line ; the posterior margin is rounded, instead of 

 wedge-shaped (as in A. glacialis) ; the sculpture is much 

 finer, and the strise more numerous, even in specimens 

 of a corresponding age and size ; and the teeth are fewer 

 and set less diagonally than in the other species. The 

 only description Which has been published of the ani- 

 mal of A. glacialis is contained in Dr. Gray's Supple- 

 ment to the Appendix to Parry's First Voyage, and is as 

 follows : — " Animal : mantle lobes separate ; foot flat, 

 compressed, subquadrate, front two cut with one or two 

 fibres from the lower edge ; trachea none." For want 



