32 UNIONID^E. 



this country, although the latter is not uncommon, in a 

 fossil state, in our upper freshwater tertiary beds. 



That part of the shell which surrounds the beaks 

 (called the " umbonal region ") is sometimes eroded or 

 excoriated in these, as well as in other members of the 

 family. I believe it is caused by the chemical action of 

 gases which are evolved from the mud in which this por- 

 tion of the shell is usually imbedded. No reliance can 

 therefore be placed on such a feature as a mark of specific 

 distinction. 



The word " Unio " is, according to Pliny, masculine. 



1. UNIO TU'MIDUS*, Philippsson. 



U. tumidus, Philipps. Nov. Test. Gen. p. 17; F. & H. ii. p. 140, pi. xl. f. 1. 



BODY greyish : mantle bordered with white ; the excretal 

 orifice being produced into a short tube, and of a brownish 

 colour with sometimes a few purplish streaks ; the branchial 

 orifice mottled with orange-brown : foot milk-white, with a 

 pale orange tint, thick and broad : gills pale grey : labial 

 palps rather broad. 



SHELL oval, very convex above, solid, rather glossy, yel- 

 lowish-brown, transversely wrinkled : epidermis rather thick : 

 beaks slightly incurved, and placed at a distance of about one 

 fourth from the anterior side : umbonal region prominent and 

 strongly plaited in a wave-like manner, the folds sometimes 

 rising into sharp knobs or tubercles : lunule lance-shaped and 

 narrow : ligament short, strong, and prominent : anterior side 

 rounded and regularly sloping towards the front : posterior 

 side gradually sloping to a wedge-like point : lower margin 

 regularly curved : inside white and nacreous, with a faint 

 tinge of blue : hinge strong ; the right valve having on its 

 anterior side a broad, thick, and bifid tooth, which is slightly 

 bent forwards, and irregularly grooved so as to make its crest 

 notched, and having on its posterior side a long and deep 

 channel or groove, formed by a double plate, to receive the 

 corresponding tooth of the other valve ; left valve furnished 

 at its anterior side with a single wedge shaped and strong 

 * Swollen. 



