32 UNIONID^. 



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 : 

 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 : ligament short, strong, and prominent : anterior side 

 rounded and regularly sloping towards the front: posterior 

 side gradually sloping to a wedge-hke point : lower inargin 

 reffularlv 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, Avhich 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 tlie other valve ; left valve furnished 

 at its anterior side yni\\ a single wedge-shaped and strong 



* Swollen. 



