292 GINGLYMACONCHA. VENERIDZ. 
with concentric strie; the interstices seulptured with fine 
strie. 
The size is the same as that of Cyclas lacustris. 
Inhabits : 
2. Pera AMNICA. 
The shells obtrigonal, smooth, sculptured with irregular, 
concentric striee ; the interstices sculptured with striz. Rather 
larger than the former. 
Inhabits 
3. Pera FLUVIATILIS. 
The shells obtrigonal, somewhat compressed, smooth, 
slender, sculptured with irregular, broad grooves; the inter- 
stices slightly compressed. Very much smaller than the pre- 
ceding species. 
Inhabits : 
4, Pera APPENDICULATA. 
The shells obtrigonal, very thin, glossy, smooth, polished, 
sculptured with strize almost regular ; their umbones furnished 
with a rounded process. Length four inches and a half; 
height two and a half lines. 
Inhabits the river Cam, near Cambridge. 
5. Pera PuLcHELLA. 
The shells obtrigonal, very smooth, glossy, polished, seulp- 
tured with regular strie. Length four and a quarter, height 
two and a half lines. 
inhabits the river Cam, and in rivulets near Cambridge. 
6. Pera GIBB. 
The shell obtusely obtrigonal, gibbous, thin, glossy, polished, 
sculptured with irregular striole. Length 5 
Inhabits 
Genus 38. CoRDULA. 
Shells cordiform ; the umbones situated behind the middle 
