MARINE MOLLUSKS — MACGINITIE 185 



from 217 feet; and 4 others (from 6 to 10.5 mm.) came from 122, 132, 

 184, and 341 feet. Eight of the specimens that washed ashore range 

 between 39 and 55 mm. in length; 1 is 11 mm., and 2 are 18.5 mm. 



One empty shell (about 57.5 by 43.7 by 29 mm.) and 1 valve of about 

 the same size, taken from Eluitkak Pass on Aug. 10, 1948, belong to 

 the var. uddevallensis. 



Other material examined: Approximately 30 specimens from 

 Norway, the Shetlands, Greenland, Cumberland Gulf, and Maine — 

 about 4 of which belong to the var. uddevallensis; also specimens from 

 Puget Sound. 



Discussion: In typical M. h'uncata the posterior end (measured 

 from the vertical from the beaks) is nearly as long as the anterior 

 end, and the posterior end is obliquely truncate in such a manner that 

 the ventral margin extends farther posteriorly than the dorsal (pi. 25, 

 fig. 1). In typical var. uddevallensis the posterior end is so shortened 

 that it is not much more than one-half as long as the anterior, and it is 

 truncate in such a manner that the posterior dorsal margin is almost 

 twice as long as the posterior ventral margin (pi. 25, fig. 3). Typical 

 var. uddevallensis is much higher in proportion to the length than 

 typical M. truncata, and the pallial sinus is much shorter in the former. 

 However, there are all types of intergradations between these two 

 extremes: In some specimens the posterior end is squarely truncate 

 and in others even somewhat rounding-truncate; there are all degrees 

 of relationship between the proportions of the length of the anterior 

 and posterior ends; and the pallial sinus varies from extremely shallow 

 to so deep that it may project anteriorly to or beyond a vertical line 

 from the beaks. The shorter, uddevallensis, form tends to be more 

 inflated in the anterior regions than the longer M. truncata. 



The var. uddevallensis is sometimes listed as a separate species, 

 and although typical var. uddevallensis differs markedly from typical 

 M. truncata, complete series of intergradations leave no doubt that 

 the former is only a variety of the latter. Foster (1946) considers 

 uddevallensis merely an extreme variant of M. truncata. The M. 

 truncata from Point Barrow (pi. 25, fig. 2) is intermediate between 

 typical M. truncata and typical var. uddevallensis. 



There has been considerable discussion regarding the differences 

 between M. truncata and M. arenaria, some workers even going so far 

 as to suggest that M. truncata is merely a variety of M. arenaria, but 

 no one who has seen the living animals would fail to distinguish 

 betv/een adults of these two species. In M. truncata a thick, tough, 

 loose, and highly wrinkled sheath of periostracum extends onto the 

 siphons (see Abbott, 1954, pi. 32 v) and covers them to the tip; in 

 M. arenaria this sheath is very thin and closely adherent and does 

 not extend very far. The pallial sinus of M. arenaria is narrower 



