336 KA.NSAS UNIVERSITY SCIENCE BULLETIN, 



appearance. The relation of the height of the shell to its 

 length is commonly about as one to three, but specimens 

 often occur in which the height is one-half or more of the 

 length. It is a peculiar fact that in spite of the variation in 

 the thickness and height of the shell the cubic capacity of 

 the valves bears a quite constant relation to their length. 

 The umboidal ratio is also fairly constant. 



U. gibbosus is an easily recognized species in spite of its 

 variation. The only forms with which it might be confused 

 are Lampsilis recta and Ptychobranchus phaseolus. The dis- 

 tinguishing characteristics are mentioned in the notes on 

 those two species. 



Unio tetralasmus Say. Plate LXXVIII, fig. 1. 



Unio tetralasmus Say, Amer. Conch., in, 1830, pi. xxill. 



Unio symmetricus Lea, Proc. Amer. Phil. Soc, iv, 1845, d. 164. 



Unio jamesianus Lea, Proc. Acad. Nat. Sci. Phila., I, 1857, p. 84. 



Shell moderate to large, moderate thickness, long elliptical, 

 slightly compressed. Anterior margin rounded ; ventral mar- 

 gin slightly bowed and forming quite a sharp angle with 

 posterior margin ; posterior margin straight or slightly 

 bowed; dorsal margin straight and forming an approximate 

 angle of 140 to 145 degrees with the posterior margin. Um- 

 boidal ratio, 0.20 to 0.30. Umbones full, decurved, and 

 marked with from five to eight rather coarse concentric 

 ridges. Anterior umboidal slope fully rounded ; lateral 

 slope moderately rounded ; posterior slope slightly angular. 

 Epidermis smooth and shining, except for an occasional nar- 

 row rough band and for the dorsum of the posterior slope ; 

 in color from light horn to dark seal-brown, sometimes 

 marked obscurely with fine rays of dull olive-brown. The 

 color of the epidermis is often arranged in broad concentric 

 but not continuous bands of light and dark ; umbones gen- 

 erally brown. Lines of growth fine, slightly imbricated, and 

 continuous. Ligament long, narrow, and black. 



Interior : Pseudocardinals thin, erect, plate-like, high or 

 low, double in left and single in the right valve, as a whole 

 very variable. Laterals thin, of moderate length, and slightly 

 curved. Anterior muscle cicatrices large and only fairly well 



