﻿MARGARITANA 517 



Margaritana margaritifera falcata Dall, Alaska, XIII, 1905, 



p. 132. — Hannibal, Pr. Mai. Soc. London, X, 1912, p. 122. 



Unio (Alasmodonta) contplanatus Middendorf'F, Sib. Reise, 



II, 185 1, Pt. I, p. 273, pi. XXVII, figs. 1-6. 



Margaritana complanata Wf.sterlund, Faun. Pal., II, Pt. 7, 



1890, p. 187. 

 Unio mongolicus Middendorff, Sib. Reise, II, 1851, p. 2yy, 



pi. XXVII, figs. 7, 8. 

 Alasmodon yubaensis Trask, Pr. Cal. Acad. Sci., I, 1855, P- 3^- 

 f Margaritana rareneliana Chenu, Man., 1859, II, p. 144, fig. 



714- 



This species has the widest distribution of any Naiad in the 

 world. It is found throughout Europe excepting, perhaps, 

 the extreme southern portion, in Northern Asia, Japan, West- 

 ern North America south to below the fortieth degree of lati- 

 tude and east of the Rocky Mountains in the upper Missouri 

 River. It seems to be missing in the North Central part of 

 North America, but is abundant in Eastern Canada, New 

 York, Pennsylvania and New England. I have examined 

 extensive series of this shell, including the Lea, the Jeffreys 

 and the general collections of the National Museum, some 

 thousands in all, and I am unable to separate the species into 

 satisfactory varieties. Usually the laterals are nearly or quite 

 wanting in adult specimens ; but occasionally they are quite well 

 developed and I have seen full grown shells from Massachu- 

 setts, the Western United States, Japan and Europe with very 

 fair laterals. In the young shells they are often developed. 

 In localities where food is abundant and conditions are favor- 

 able the shells are large, clean, comparatively high and straight, 

 the base being full, and this development has received names 

 in Europe, but it occurs throughout the range of the species. 

 Other specimens, which have not had abundant food or favor- 

 able environment, are small, stunted and decidedly arcuate. 

 Old shells often become arcuate, as is the case among many 

 Naiades. Among the shells of the Western United States 

 the nacre is often deep purple or sometimes salmon red. I 



