168 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 109 



Jensen (1912) writes that var. striata is by far the most predominant 

 form at western Greenland, with var. globosa occurring frequent!}^, the 

 typical form and var. warhami being comparatively rare. He also 

 states that A. montagui "becomes elongated on the whole in the same 

 degree as the marine climate becomes more severe," the t3^pical form 

 being found in the Faroes and southwest Iceland, the var. striata 

 beginning to appear at the northern end of western Iceland; at western 

 Greenland the var. striata predominates and the more elongate 

 warhami begins to appear, whereas at eastern Greenland the var. 

 warhami is almost the only form. 



-:■ The findings at Point Barrow agree with Jensen's observations, for 

 the great majority of the A. montagui from there belong to the var. 

 warhami Hancock (pi. 22, figs. 11, 12) and there are no true forma 

 typica. A much lesser number can be assigned to the var. fabula 

 Reeve, a few to the var. striata Leach (see pi. 22, fig. 14), and one or 

 two approach the dimensions of var. globosa Moller. The largest speci- 

 men, from a depth of 246 feet, measures 35 by 29.6 by 17.1 mm. It 

 has the periostracum of A. montagui and the shape of var. striata but 

 the shell is exceptionally large for either the typical form or the variety. 

 The next largest is a var. warhami measuring 27.2 by 21 by 11.4 mm. 

 Jensen found that accompanying the elongation of the form there is an 

 increase in maximum length from around 19-20 mm. at the Faroes 

 and western Iceland to 23-26 mm. at eastern Iceland and Greenland. 

 The majority of the larger specimens from Point Barrow come within 

 the 23-26 mm. range. Specimens from any large haul usually have 

 representatives of the varieties warhami, fabula, and striata, with 

 intergrades (pi. 22, figs. 15, 16). 



There is as much variation in sculpture as in shape. The most 

 common sculpture consists of prominent, evenly spaced, concentric 

 riblets (with about equal interspaces) in the umbonal region, wdth less 

 prominent, irregularly spaced, concentric riblets throughout the lower 

 two thirds of the shell or with fine lines that give the shell a smooth 

 appearance, but in some shells the umbonal sculpture extends to the 

 ventral margin, and in others the umbonal sculpture is only slightly 

 more prominent than that throughout the remainder of the shell. 



The color ranges from a 3/ellowish tan to a dark chestnut brown. 

 In some shells a black deposit covers part or sometimes nearly all of 

 the periostracum. The latter sometimes cracks and peels away from 

 the shell and the shell itself sometimes cracks. 



Like A. borealis, this species closes so tightly that it is difficult to 

 insert a razor blade between the valves. 



Yearly age lines are not so conspicuous in this species as in some of 

 the other Point Barrow shells. A shell 7.3 mm. long appears to be 



