THE NAUTILUS. 77 



ON THE CLASSIFICATION OF THE UNIONIDAE. 



BY CIIAS. T. SIMPSON. 



In the August and September numbers of the Nautilus, Dr. von 

 Ihering takes exception to the classification of the Unionidse pro- 

 posed by me in the Synopsis, claiming that it is based essentially on 

 the marsupia. 



He has apparently overlooked the fact that it is founded not 

 merely on the characters of the marsupia, but on the more obvious 

 anatomical characters, as well as those of the shell and the beaks. 

 It agrees with what I believe to be the development of the family from 

 the earliest and simplest forms to the latest and most highly organ- 

 ized. 



So far as the classification of the Diplodontinse is concerned we 

 essentially agree. This I divided into two supergeneric groups 

 founded on characters of the beak sculpture and shell, and not on 

 those of the marsupia. Von Ihering agrees with me that the earliest 

 uniones probably had radial beak sculpture. I have examined the 

 animals of a large number of the South American and Australasian 

 uniones and in all cases where they were gravid the embryos filled 

 the inner gills alone, forming a smooth pad, the ovisacs not being 

 separated by sulci. I have examined a few specimens of the forms 

 with zigzag radial beak sculpture (Rosanoramphus), and found in 

 the gravid ones that the marsupia filled the inner gills only. It may 

 be, and probably is the case, that in rare instances among the 

 Hyrianje there are a few embryos in the outer gills. I know of no 

 special characters employed in the classification of large groups 

 which do not vary somewhat, but I believe it still to be a fact that 

 in the Hyrianse the embryos are almost invariably contained in the 

 inner gills only, that in the Unioninse they are found in the outer, or 

 (in the Tetragence) all four of the gills. 



In the South American and Australasian Uniones we have the 

 radial beak sculpture with simple shells and the embryos contained 

 within the inner gills, the ovisacs not being separated by any ex- 

 ternal markings. I believe that these are the simplest and lowest 

 forms of Unione life, that they have descended almost unchanged 

 from the earliest forms of the family. The fact that they occupy the 

 Southern Hemisphere exclusively while the Unioninae, containing 



