326 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 1 8, 



undulatus. We must dismiss this for the present, for we do not 

 know much about its taxonomic standing and its distribution. This 

 may be nothing but a local form of Strophitus edentulus, and then 

 it would have to the latter the same relation as Lampsilis ochracea 

 has to L. cariosa (the former is the tidewater form of the latter). 



As a whole, the Atlantic fauna should be regarded as an offshoot 

 of the fauna of the interior basin, with the exception of Margaritana 

 margaritifera. It does not possess any very strongly marked types 

 of its own, but all may be traced back to western types. How- 

 ever, there are different elements on the Atlantic slope, which ap- 

 parently reached their present range by different ways, and probably 

 at different times. The greatest independence is shown among 

 those which are found in the southern section of the Atlantic slope, 

 and there is an indication of the development of a secondary center 

 of dispersal in this region, producing a few characteristic types, 

 more remote in their affinities from the forms of the interior basin. 

 The other forms are generally more or less closely connected with 

 western species, in fact, clearly are representative forms of them. 



Chapter 3. 



Distributional Facts in other Freshwater Animals. 



Before we advance further in our attempt to study the mutual 

 relations of the eastern and western freshwater faunas, it is well to 

 compare a few other groups with the Najades, in order to ascertain 

 whether there are parallel cases to those described above. 



I. SPHAERIID^E. 



For the identification of my material I am indebted to V. Sterki. 

 Although I have collected a great many Sphccriidce from the streams 

 of Pennsylvania, West Virginia, and Virginia, my collections are by 

 no means complete. Nevertheless, as far as they go, they serve to 

 confirm the well-known fact, that with regard to these small shells, 

 the Alleghenian divide does not form an important faunistic bound- 

 ary. Thus the Sphccriidce distinctly differ from the Najades, and 

 undoubtedly must have been subject to other laws. 



