128 CATALOGS OF SPECIES OF PIS1DIUM. 



sloping fairly steeply to a point, well 1 flow the general 

 level of the hinge- plate and revealing the ligament -pit, 

 distal ridge very steep at first, then sloping gradually. 



Dimensions: Long. 3'2, Alt. 2'7, Crass. 2 mm. 



The specimens described come from the Pleistocene (Campinien) 

 at. Soignies, Belgium [Mas. Hist. Xut. Brussels], and were sent me 

 with the other Belgian fossil species at the instance of Mons. E. 

 Vincent, in honour of whom the species is named. 



The marked feature of this unmistakable form is the extraordinary 

 depression of the ligament-pit, which can only be properly seen 

 when the hinge is viewed from across the ventral margin of. the 

 shell. 



So far the only species seen at all resembling it is the living 

 P. steii'arti, Preston (140, p. J 16), taken at an altitude of 14,500 

 feet at High Hill Gompa, Gyantse Valley, Tibet. 



