MARINE MOLLUSKS — MACGINITIE 161 



and its varieties the central portion of the shell between the posterior 

 and anterior striations is much smoother (pi. 18, fig. 10) than in M. 

 cor)"ugatus; in the former the periostracum of this central area has 

 delicate, simple, transverse lines but in AI. corrugatus it is wi-inlded 

 and shagi'een-like. In adults, both the posterior and anterior stria- 

 tions are more numerous in Al. niger than in the other two species, 

 and the posterior striations of M. corrugatus are more numerous than 

 those of Af. discors var. laevigatus f. suhstriatus. 



Color, on which certain varieties and species have been based in 

 the past, is a very unreliable characteristic in this group. In the 

 specimens examined, all gradations of color from a pale oHve green 

 through browns to a shiny black (Plover Bay) were found. Several 

 are a mixture of brown and black. Very j^oung specimens from Point 

 Barrow are a pale yellowish green. 



Juvenile specimens of M. discors and its varieties are difficult to 

 separate from those of M. corrugatus and AI. niger, for all of them 

 have both anterior and posterior striations on the shells. The j^oung 

 of AI. discors var. laevigatus are usually more inflated than those of 

 A/, corrugatus, and the latter are more inflated than those of AI. niger. 

 The umbos of juvenile M. corrugatus are prominent (pi. 21, fig. 4). 

 The ventral margin of juvenile AI. corrugatus is fairly straight (pi. 21, 

 fig. 4), that of M. niger is somewhat convex (pi. 21, fig. 6), while that 

 of AI. discors var. laevigatus is somewhat more arcuate and the first 

 posterior striation projects somewhat (pi. 21, fig. 5). The dorsal 

 margin of juvenile AI. niger ascends posteriori^'- more rapidl}'' from 

 the umbos (pi. 21, fig. 6) than in AI. discors var. laevigatus (pi. 21, 

 fig. 5). 



The degree to which the posterior striations fade out in the varieties 

 of AI. discors as growth proceeds farther from the umbos is highly 

 variable. Some shells were observed in which no indication of stria- 

 tions could be seen bej^ond the first j'ear growth Ime and in others the 

 striations persisted, though less marked, to the second year growth 

 line. In others faint striations could be seen in adult specimens and 

 in a few the striations fade out in the middle portion of the posterior 

 end and appear again near the margins. 



The Point Barrow specimens belong to the var. laevigatus and a 

 few of them could be assigned to the forma suhstriatus. This sub- 

 stantiates Jensen's (1912) statement that he has not found a true 

 Ad. discors in the Arctic. I am indebted to Dr. Thorson and Mr. 

 W. K. Ockelmann for sending me specimens of true A\l. discors, 

 AI. discors var. laevigatus, and AI. discors var. laevigatus forma suh- 

 striatus, as well as specimens of AI. corrugatus. 



Thorson (1935) states that in eastern Greenland AI. discors var. 

 laevigatus draws together blades of Fucus or Laminaria hj means of 



