THE NAUTILUS. 21 



distending. No secondary watertubes developed within the marsu- 

 pium. Glochidia rather small, either semioval or semielliptic, with- 

 out hooks, or subtriangular, with hooks. — These forms have a short 

 breeding season. 



The typical genus is Unio Retzius, of which the European species 

 Unio pictorum (Linnaeus) is the type. 



Unio pictorum (Linnaeus). A large number of specimens is at 

 hand, from various parts of Germany and Hungary, representing 

 sevei'al " forms." 



The anatomy is typical for the family : the supraanal is well sepa- 

 rated from the anal by rather long mantle-connections. The mar- 

 supium is formed by the outer gills alone, and according to the struc- 

 ture of the edge of the gill, it does not seem capable of distending, 

 when charged. 



No gravid females are at hand, but according to various authors, 

 the breeding season is short, and the glochidia are subtriangular, 

 with hooks. 



This latter character distinguishes the European genus Unio from 

 the genus bearing the same name in North America. In addition, 

 in the latter, the mantle- connection between the anal and supraanal 

 is generally shorter, and the shell chiefly the beak sculpture, is fun- 

 damentally different. Thus we cannot place the North American 

 species in the same genus, and I have concluded to use for them 

 the name Elliptio Rafinesque. 



Unio tumidus Retzius (about 10 specimens from Germany and 

 Hungary at hand); U. crassus Retzius (many typical specimens from 

 Thuringia), and the probably conspecific forms : batavus Maton & 

 Racket (5 specimens from Germany and Hungary), and consentaneus 

 Rossmaessler (one male from Bavaria), have essentially the same 

 anatomical structure as U. pictorum. A specimen of U. batavus 

 from Hungary had the outer gills partly filled with eggs. No lateral 

 water-tubes were observed. 



Subfamily : Anodontin^ Ortmann. 



Supraanal always well separated from the anal opening, generally 

 by a very long mantle-connection. Marsupium formed only by the 

 two outer gills, when charged greatly swollen, and an extra thickness 

 of tissue at the edge permits them to dis'end. Within the water- 

 tubes of the marsupial gills, lateral (secondary) water-tubes, lying 



