310 Mr Jenvns's Monograph on the 



Pisidium obliquum, Pfeifer, 124. /. 5. / 19, 20. 



Cyclas amnica, Turt. Conchyl Brit. 250. t. 11./ 15— Fleming, 453. 

 Turt. Man. 15. / 5. 

 Var. fi. 



Sulcis profundius exaratis. 



Pera fluviatilis, Leach, MSS. in Brit. Mus. 

 Var. y. 



Striis levius impressis. 



Pera Henslowiana, Leach, MSS. in Brit. Mus. 



Animal album, siphone paulo variabili; nunc abbreviato, subconico, apice 

 oblique truncato; nunc elongate-, cylindraceo, apice plus minusve 

 recurvo. 



Testa paul6 variabilis, ovalis, vel oblique trigona, distincte inasquilateralis, 

 ventricosa, antice planiuscula, pulchre striata hie et illic sulcis pro- 

 fundioribus; cinerascenti-fusca, raaculis et zona marginali lata palli- 

 dioribus, interdum nitide lutescentibus ; rarius omnino fuscescens aut 

 lutescens ; intus csrulesoens : umbones obtusi, sorde ferruginea \\t in 

 prajcedentibus sajjie incrustati. 



This species, which was first published as British by Dr Maton 1. c. 

 is at once distinguished from all the others in this genus by its very 

 superior size. It is not uncommon in rivers and gently running streams, 

 residing wholly at the bottom, and being partially buried in the mud, 

 but I have not often observed it in perfectly stagnant waters. Vars. 

 /3 and y were sent to Dr Leach by Professor Henslow from the 

 neighbourhood of Cambridge. The former of these gentlemen con- 

 sidered them as distinct species, and they accordingly stand in the 

 collection at the British Museum, under the above names; but I am 

 perfectly satisfied that they are mere varieties, differing in nothing 



