Occasional Papers of the Museum of Zoology 9 



The hinge-armature is similar to that of juvenile specimens, 

 and the color of the nacre is that usual in medium-sized indi- 

 viduals — i. e., lavender with copper tints. However, the shells 

 are solid, somewhat inflated, and much eroded at the beaks, 

 and have every external appearance of mature individuals. 

 One (fig. 5) is quite sinuate on the ventral margin. They 

 measure : 



Length 



Height-index 



Diameter-index 



Fig. 4 



Fig. 5 

 Another 



SO mm. 

 54 mm. 



55-5 mm. 



B. plexus 55 



U. pigerrimxis 59 



60 (30 mm.) 



61 {23 mm.) 

 58 {2,2 mm.) 



64 



65 



39 (19.5 mm.) 

 39 (21 mm.) 

 42 (23 mm.) 



44 (Simpson, 1914) 

 46 (C. and F., 1894) 



Unio coloratiis has every appearance of a smooth specimen 

 of this species. The question of U . plicatulus has already been 

 discussed. U . pigerrimns easily falls within the range of varia- 

 tion of this and the following form. 



The section Sphenonaias C. and F. (1894) (type U. lieb- 

 inanni Ph.) is used here to include, besides the type, the more 

 EUiptio-like forms of Psoronaias (type U. psoriciis Morelet). 

 The typical forms with higher uinbones have much more the 

 appearance of soine of the southern species, placed by Simp- 

 son (1914) in Quadrula, but which were also included in the 

 section Psoronaias, as originally described. The section Bary- 

 naias C. and F. (1894) is described in Part VII of the "Moll, 

 terr. et fluv. de Mex.," with U. pigerrimns as the single example, 

 while in Part VIII that species is put under Psoronaias and 

 U. sallci is listed as the sole example of the former section. 

 Barynaias is thus a synonym of Psoronaias. Von Martens 

 (1900) mistakenly calls U. sallci the type of Pachynaias C. 

 and F. Barynaias is also a synonym of Sphenonaias, as used 



