378 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



form) in northwestern North America and in absolutely the same 

 form in Iceland and parts of Europe and Asia. The distributional 

 facts have been summarized by Walker (1910), and as to the origin 

 of the distribution he draws the conclusion (/. c, p. 139) that the 

 presence of this species in northeastern North America is best ex- 

 plained by the assumption that it immigrated, probably in late 

 Tertiary times, from Europe by a land-bridge over Iceland and 

 Greenland. 



I accept this fully. Also the idea of Walker, that the Glacial 

 epoch restricted the range of this species, must be accepted. In 

 fact, we are to regard the present station in Pennsylvania as the last 

 remnant of the Glacial refugium of this species, just in front of the 

 terminal Moraine. Here it survived and the present distribution is 

 largely a Postglacial re-occupation of lost territory, 28 and in this it 

 fully agrees with the other Atlantic forms, chiefly the northern ele- 

 ment. It differs, however, from the latter in its ecological prefer- 

 ences : Margaritana is a form of cold water and is averse to limestone. 



Thus it is evident that Margaritana is a stranger among the other 

 N a jades of the Atlantic side, in fact, it is an element of the North 

 American fauna which stands by itself and has been subject to en- 

 tirely different laws in its distribution. It is true, there is a shell 

 in the interior basin which is allied to it, but only remotely so, be- 

 longing to another genus : Cumberlandia monodonta ( Say ) . Another 

 one is Margaritana hembeli (Conrad) from southern Alabama and 

 Louisiana. 29 Both of these do not seem to have any direct genetic 

 connection with M. margaritifera and are probably relics of a former 

 more general distribution of this most primitive and archaic group 

 of N a jades, undoubtedly reaching back in their history far beyond 

 the other Najades and far into Mesozoic times. 



Eurynia nasuta. 



On the Atlantic side this species is found from the Delaware 



2S It is doubtful, whether all of the present range was regained from this 

 Pennsylvanian stock; it is quite possible, that there were other refugia, sit- 

 uated on the former seaward extension of the present coast. The Pennsyl- 

 vanian refugium is the only one, which has been positively ascertained. 



29 The so called Margaritana decumbcns (Lea) of Alabama is an ex- 

 tremely doubtful form in every respect (see Walker, /. c, p. 128). 



