306 KANSAS UNIVERSITY SCIENCE BULLETIN. 



variable ; umbones full and swollen, decurved and ornamented 

 with about seven slightly double-looped ridges. Anterior 

 umboidal slope abruptly rounded ; posterior slope gradually 

 rounded but flattened somewhat near the posterior margin in 

 the female. Epidermis smooth, often cloth-like, with occa- 

 sional dark continuous lines of growth, in color from dark 

 horn to honey-brown ; young specimens a sage-green and 

 marked with numerous narrow rays of dull green, which are 

 often obscurely present in the adult shell. Ligament rather 

 long, stout, chestnut brown. 



Interior: Pseudocardinals heavy, very variable, generally 

 high, erect, columnar or pyramidal, the posterior left often 

 long and lamellar, double in the left and single in the right 

 valve. Laterals of varying length and curvature, coarse and 

 high. Anterior adductor cicatrix of moderate size, deeply 

 excavated, set in front of the pseudocardinals. Posterior scars 

 of moderate size, well impressed, often confluent. Pallia! line 

 generally impressed its entire length. Dorsal cicatrices 

 numerous round pits in the cavity of the umbones. Cavity 

 of beaks and shell moderate. Nacre white, slightly iridescent 

 posteriorly. 



0. ellipsu is found in the St. Lawrence drainage, and as far 

 south as Tennessee and Arkansas in the Mississippi drainage. 

 It has been found in all the drainage basins of the state, but 

 it is most common in the Kansas river and its larger tribu- 

 taries. It is here that, according to Call, it reaches its maxi- 

 mum development. I have seen shells having a length of three 

 and three-fourths inches. 



It is a lover of water of moderate depth and of sandy river- 

 beds. Before the great floods of 1903— '04 it could be found 

 in large numbers on the sand-bars near Lawrence. Its strong 

 musculature and smooth shell enable it to move with compara- 

 tive rapidity, and I have traced its furrow for fifty yards on 

 sand-banks in the Kansas river. 



