MARINE MOLLUSKS — MACGINITIE 183 



Melville Island; 1 small specimen from Point Barrow; 1 from Icy 

 Cape and 1 from between Icy Cape and Cape Lisburne; and 6 or 7 

 from Plover Bay. Also Ball's figured type and type lot of M. krausei 

 from off Icy Cape, Alaska, 7-15 fathoms. Over 50 specimens labeled 

 M. moesta from localities ranging from Icy Cape to Bering Strait to 

 the Pribilofs, the Aleutians, and the Shumagins; also from Plover 

 Bay and Kamchatka. 



Discussion: Jensen (1905) places M. krausei Dall in the synonymy 

 of M. moesta. Ball's type lot of M. krausei (pi. 21, figs. 1-3) consists 

 of 4 small shells and 1 valve, ranging in length from 6.8 to 12.7 mm., 

 and 1 dead shell (the figured type, pi. 21, fig. 1) 23.1 mm. in length. 

 The small shells of M. krausei are very similar to the 7 small shells 

 and 1 valve from Point Barrow, but the shell of the figured type (pi. 

 21, fig. 1) is more pointed and more extended posteriorly and less 

 extended anteriorly, so that the umbos are not so near the posterior 

 end as in the smaller shells and in the typical adults of M. moesta. 



Although Dall described typical adult M. moesta as M. oneilli, the 

 50 or more smaller specimens mentioned above were identified as M. 

 moesta. That Dall himself may have entertained some doubt as to 

 the specific validity of M. krausei is suggested by the fact that he 

 (Dall, 1921, p. 47) gives the following reference for M. moesta De- 

 shayes: 'Troc. U. S. Nat. Mus., vol. 23, pi. 4, fig. 8." This reference 

 is for the original description of M. krausei and in this paper Dall 

 (1921) does not even mention M. krausei. 



The 7 small specimens (see pi. 24, figs. 2, 3) from Point Barrow 

 correspond fully with the 50 smaller specimens labeled M. moesta. 

 Some of the latter shells have a thin, transparent periostracum, others 

 a yellowish olive-green ; those from Point BaiTOw have the thin trans- 

 parent periostracum. Those from Kyska Harbor and Amchitka 

 Island are somewhat more inflated than usual; others from these two 

 localities, as well as a few from the Shumagins, are somewhat higher 

 in proportion to the length than others. Several small specimens of 

 M. moesta from Greenland (sent to me by Dr. Thorson) have a yellow- 

 ish olive-gi-een periostracum and are high in proportion to the length 

 (see table 6). 



The 8 adult specimens from Point Barrow correspond fully with 

 Dall's M. oneilli and they also correspond fully with Deshayes' 

 description of M. moesta. W. K. Ockelmann of Copenhagen, who 

 has compared them with specimens of M. moesta from Greenland, 

 states that they are unquestionably M. moesta. Except on the 

 anterior end, these shells have a brownish concretion along the margins 

 (pi. 24, fig. 1), and the periostracum extends onto the interior surface 

 for as much as 2 mm. (pi. 23, fig. 10). 



