32 UXIONID^. 



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 s Dccific 

 distinction. 



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



1. Unio tu midus"^, Philippssou. 

 U. fumidus, PhiHpps. Nov. Test. Gen. p. 17 ; F. &H. ii. p. 140, pi. xl. f. I. 



Body greyish : mantle bordered with white ; the excretal 

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

 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 : (jiUs pale grey : labial 

 jyaljps rather broad. 



Shell oval, very convex above, solid, rather glossy, yel~ 

 lo wish-brown, transversely wrinkled : epidermis rather thick: 

 heahs 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: lir/ament short, strong, and prominer^ : anterior side 

 rounded and regularly slojiing toward'^ Ihe front: posterior 

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

 regularly curved: inside white and nacreous, with a faint 

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

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

 bent forAvards, 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. 



