508 PROCEEDINGS OF TRE NATIONAL MUSEUM. vol.xxii. 



America to New Zealand and Australia, and slight changes in the way of 

 progress are shown there. The beak sculpture is still radial, but the bars 

 are decidedly curved toward each other below; they become slightly 

 broken or nodulous, and each set is generally removed from the other a 

 little, so that a space in the middle of the young shell is smooth. The 

 young are contained in the inner gills, though Suter reports a few in 

 the outer gills of one or two species and in all these Southern Hemi- 

 sphere forms the shell has a dull color, and is nearly or quite rayless. 

 Another migration took place to Southeastern Asia, and from there 

 there was another to tropical Africa, possibly from Southern India 

 over an old but now lost landway. In the forms of this region the 

 beak sculpture becomes irregularly and variously zigzag radial, the 

 surface is nodulous in many cases, and the shell is often bright colored 

 and painted. Some of the recent species, such as Unio crisinsidcatus, 

 show nearly strict radial sculpture all over the shells; others which 

 I have placed in the genus Lamellidens have almost such sculpture as 

 is seen in the beaks of the Australian forms. In others tlie sculpture 

 becomes slightly nodulous. Eeasoning from analogy, and the few 

 gravid specimens I have examined of these forms with the zigzagged 

 beak sculpture, I presume that they all carry the young in the inner 

 gills, though it is quite probable that some of them may have them in 

 all four gills, and this would be a step in an upward direction. Some 

 of these Asiatic and African forms with zigzag sculpture are quite 

 short, inflated, and solid, and, in general, show characters strongly 

 allied to those of our heavy Mississippi Valley forms (the latter of 

 which I place in Quadrula), the various members of the genus Parreysia, 

 for example. 



It is quite probable that from such forms the genus Quadrula sprung; 

 that it developed in Asiatic streams, where it still seems to be repre- 

 sented. In the Tertiary strata of eastern Europe and in parts of 

 Asia this genus seems to be abundantly represented. It is a little dif- 

 ficult to say from some of tlie living Asiatic forms whether the beak 

 sculpture should be called sharply, doubly looped, or zigzag radial, 

 and the same thing may almost be said of some of our North American 

 forms. There probably existed at that period an old land way across 

 from northeastern Asia to northwestern North America, and one which 

 lasted a long time, or it may have been submerged and then reappeared 

 again, for through long ages this has apparently been a highway for 

 migrating Unionidfc. I think it not unlikely that the immediate progeni- 

 tors of the magnificent and diversified series of Uniones found fossil in 

 the Laramie beds came over from Asia among the earlier migrations; 

 for it is a significant fact that among the Uniones of the Laramie for- 

 mation in the United States we have a number of species which in general 

 form closely resemble these recent Asiatic Parreysias, and that their 

 beak sculpture is decidedly zigzag radial, just as it is to-day in these 

 oriental forms. Associated with these Laramie species are others in 



