British species of Ci/clas and Pisidinm. 311 



but the depth and number of the longitudinal furrows and stria2, than 

 which no characters can be more variable. The shell of this species, 

 like that of the others belonging to this section, is frequently incrusted 

 over with a kind of ferruginous earth, which prevails chiefly at the 

 posterior extremity. Perhaps in the present instance this circumstance 

 is connected with the habits of the animal ; which usually having the 

 anterior half of the shell fixed in the mud, the posterior and exposed 

 portion receives all those finer particles of the soil which drifting 

 downwards with the stream, are thereby deposited on its surface. — 

 The young of this species are readily distinguished from the two last, 

 by their more compressed shell, with the umbones scarcely at all promi- 

 nent and the stride more distinct. 



The foregoing list includes all the British species belonging 

 to the above genera which I have been able to identify satis- 

 factorily. T possess one or two other shells which appear dif- 

 ferent from all hitherto described, but not having seen a sufficient 

 number of specimens to judge of their true characters, I should 

 not feel authorized in admitting them as really distinct. — I men- 

 tion the circumstance, however, for the sake of exciting further 

 enquiry upon the subject. 



LEONARD JENYNS. 



SwAFFHAM BULBECK, 



Nov. 14., 1831. 



