THE NAUTILUS. 21 



gills, the branchiae when distended with them being perfectly smooth 

 outside, and looking like pads. There seem to be two great groups 

 of these forms, one characterized by simple, oval or oblong shells 

 destitute of any strong sculpture, and probably carrying the young, 

 as a rule, only in the outer branchiae, and this includes in the 

 United States such forms as Unio gibbosus Bar., U. tetralasmus Say, 

 U. buckleyi Lea, U. crassidens Lam. and U. complanatus Sol., and 

 these are probably closely related to the European species. The 

 other group has short, rather solid, often inflated shells, with a wide, 

 heavy hinge plate, and it includes nearly all the pustulous, and all 

 the plicate sculptured forms. Lea found the inner and outer gills 

 filled with embryos in four of these species: U. multiplicatus Lea, 

 subrotundus Lea, kleinlanus Lea, and rubiginosus Lea, and it is 

 probable that, under favorable conditions, all or most of these spe- 

 cies carry young to some extent, in the inner as well as the outer 

 gill, though so far as I have observed the inner gill is never so com- 

 pactly filled as the outer, and it is quite probable that with unfav- 

 orable conditions the former may not be used as a marsupium. 



Besides these there are a few aberrant forms which may be, as 

 Wetherby has suggested, " geological remnants," such as Unio 

 phnseolus Hild., U. irroratus Lea, and U. cornutus Bar., having 

 remarkable modifications of the branchial uterus or marsupium. 

 These three species will probably have to stand as the types of as 

 many genera. 



But little is known concerning the anatomy of the foreign Unios. 1 

 The soft parts of all the European species have been examined, I 

 believe, and descriptions which go into the minutiae, so far as color 

 and trifling peculiarities of form are concerned, have been pub- 

 lished, but which give no idea of vital characters or structure. 

 From all that I can learn the anatomy of the European forms is 

 very much like that of thecircumboreal Unio margaritiferus, which 

 is much like that of Unio gibbosus, crassidens, tetralasmus and the 

 like. Of the Oriental and African forms I know almost nothing. 

 I have examined the soft parts of gravid specimens of Unio gabo- 

 nensis Kuster from Tropical West Africa, and found that in them 

 the embryos filled the inner branchiae alone. 



It has been surmised that there was a close relationship between 

 the Australasian Unios and those of South America. The shells of 

 the species of the two faunas agree very closely in all characters ; 

 in being destitute of rays, and having a uniform olive-green epider- 



