CLASSES, ORDERS, ETC. 



61 



nearly central; ligament short, not visible from the outside. 

 Length |th inch; width |-th inch. Marshes and ditches. 



P. pusillum? 



(d) Shell suborbicular, in the upper part rather tumid, in all 

 the other parts compressed, thin, very glossy ; ligament short, not 

 visible from the exterior ; muscular impressions distinct. Length 

 j^th inch ; width |th less. Ponds and pools. P. nitidum* 



(e) Shell very small, oblong, thin, ventricose, deeply striated, 

 yellowish-white or pale horn-coloured ; umbones blunt, prominent, 

 very excentric; ligament nearly invisible; muscular impressions 

 indistinct. Length j^th inch ; width Jth inch. Ponds and pools- 



P. milium. 



Family II. Unionidae. 



Shell large, oblong, equivalve, inequilateral. Mantle lobes free 



Fig 27. Unio pictorum. 



all lound, except at the posterior edge, where they form two 

 orifices, which are somewhat awl-shaped. 



3 v. venlrosa, shell slightly more trigon-shaped, more ventricose, v. grandis, 

 much larger, v. obtusalis, shell smaller, much more tumid, umbones sharp and 

 pronounced. 



8 v. splendens, lemon-coloured. 



