GINGLYMACONCHA. VENERIDZ. 291 
4. Cycuas STAGNICOLA. 
Shells subrhomboidal, semipellucid, reddish, concentrically 
striolated ; the umbones slightly prominent and black. Length 
four-eighths of an inch; height three-eighths of an inch. 
Cyclas calyculata, Lam. Hist. Nat. des Anim. v. 559, [. 
This species is common in marshes and ponds in most parts 
of Britain. 
5. CyYCLAS ALATA. 
The shells very smooth, sculptured with concentric fine 
striee, winged above at each side; the umbones very promi- 
nent. 
Inhabits Campsey-ash in Suffolk, in ditches. 
The size exactly that of Cyclas lacustris. 
Genus 36. EUGLESA. 
Shells spherical; the umbones central or situated a very 
little behind the middle of the shells; the hinge with two 
simple teeth in each valve; the lateral lamellee short, com- 
pressed and lamelliform. 
The tracheal tubes are exserted; the under one is largest, 
with its aperture open. 
1. Eveiesa HENSLOWIANA. 
The shells trigonal, gibbous, thin, polished, sculptured with 
irregular strie. 
Inhabits the river Cam, and in marshes near Cambridge. 
Genus 37. PEeRA. 
Shell more or less oblong, behind obtuse, umbo behind the 
middle of the shell, large teeth, two equal, very small in each 
valve, hinder of left valve lobed, lateral lamella elongo-com- 
pressed, lamella shaped in one valve simple, in the other double. 
Animal syphons simple, equal, upper exsertable. 
1. Pera HENSLOWIANA. 
The shells obtrigonal, very smooth, polished, sculptured 
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