FRESH-WATER MUSSELS. 199 



mud ; they do so also in winter. Their food consists 

 of entomostraca and minute animals. They much 

 resemble mussels, but the chief difference appears to 

 be in the position of the head and ligament, which 

 is external in the Unionidce and internal in the 

 Mytilidcz, and the beak is in the former further from 

 the anterior end, while in the Mytilidcz it is nearly 

 terminal. There is a wide field still open for discovery 

 in the habits of the Unionidcz, as not much has been 

 done in detail by British naturalists. 



Unio tumidus, Philipsson, has the body grayish, 

 the mantle bordered with white ; the excretal orifice is 

 a short tube, brownish, with sometimes purplish streaks. 

 The branchial orifice is mottled with orange-brown, 

 the foot milk-white, with a pale orange tint thick and 

 broad, the gills pale gray. The length of the shell 

 is i'5, the breadth 3. The form is oval, convex 

 above, wrinkled, glossy, yellowish-brown, the epider- 

 mis thick, the beaks slightly incurved at a distance of 

 one-fourth from the anterior side, the umbonal region 

 strongly plaited in a wave-like manner, the folds 

 sometimes rising into tubercles, the ligament strong 

 and short ; the anterior side is rounded and sloping 

 towards the front, the posterior side slopes to a 

 wedge-like point, the outside is white and narrow, 

 with a tinge of blue ; the right valve on the an- 

 terior side has a broad, thick bifid tooth, slightly 

 bent, grooved so as to make its crest notched, having 

 on its posterior side a groove to receive the tooth or 

 lower valve ; the left valve has a similar tooth wedge, 

 shaped to fit the double tooth of the right valve, into 

 which it locks. This valve has also a plate on the 



