166 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 109 



cretion on portions of the shell, especially the anterior end. There 

 is great variation in the proportion of the length to the height, and 

 in the general shape of the shell: 1 (44.8 by 35 by 14 mm.) from 

 Eluitkak Pass is typical in both shape and dimensions; another (40.8 

 by 31.9 by 12.6 mm.) from Eluitkak Pass (Aug. 1, 1950) is an elongate 

 shell in which the posterior end is more pointed and extended than 

 usual (pi. 22, fig. 6); still another (42.3 by 36.7 by 12.6 mm.) from 

 Eluitkak Pass (Aug. 10, 1948) has both anterior and posterior ends, 

 as well as the ventral margin, slightl}'^ more rounded than usual 

 (pi. 22, fig. 2). In another specimen (40.6 by 34.4 by 17.1 mm.) from 

 Eluitkak Pass the posterior end is shortened and truncated, the 

 result of habitat and the vicissitudes of life, and the umbos are com- 

 pletely eroded away. In a specimen from 152 feet the shell is box- 

 like along the ventral margin (also see Cardita crassidens) . Some of 

 the variations in shape and proportions are shown by Soot-Ryen 

 (1939, pi. 1, figs. 1-3), and variations in sculpture as well as shape 

 are described and figured by Jensen (1912, pi. 4, figs. la-f). 



In many of the shells the concentric ribs around the umbos do not 

 continue for more than 7 or 8 mm. before being replaced by fine con- 

 centric lines, but in others these ribs may extend toward the ventral 

 margin for a distance of 12 mm. or more. In most the transition 

 from ribs to fine lines is abrupt, but in some it is less marked. 



The hinge is also a highly variable character, the teeth and fossae 

 vary in size and shape, and minor protuberances may be present or 

 absent. The chalkiness characteristic of many Arctic shells often 

 makes the teeth appear large and coarser than usual. Ridges, de- 

 pressions, and fossae on the inside of the shell also vary greatly in 

 size and shape. 



Shells examined from other localities exhibit the same variations as 

 the ones from Point Barrow, many having the convex lower border, 

 others with the posterior end produced, and others with a shortened 

 posterior end. Shells from the same locality may exhibit all of these 

 variations, or in any combination, and all types of intergrades are 

 found. 



Growth cessation lines are indistinct in this species, but shells from 

 45 to 50 mm. long are probably from 6 to 8 years old. 



Distribution: MacFarlane Bay near the mouth of the Mackenzie 

 River, Point Barrow, south to the Aleutians and Prince William Sound, 

 the Sea of Okhotsk, also northern Japan; in the Atlantic area south to 

 Massachusetts and Rhode Island; Greenland and Iceland; Franz 

 Josef Land, Novaya Zemlya, and northern Europe south to Bergen, 

 Norway. 



