X. A Monograph on the British species of Cyclas 



and Pisidium. 



By the Rev. LEONARD JENYNS, MA. F.L.S. 



AND FELLOW OP THE CAMBRIDGE PHILOSOPHICAL SOCIETY. 



[Read Nov. 28, 1831.] 



The genus Cyclas of Bruguiere was instituted for the reception 

 of certain species of "Bivalve Mollusca" inhabiting fresh water, 

 which were associated by the older Linnaean authors, either with 

 Cardium or Tellina. Only three of these shells appear to have 

 been distinctly known to Montagu as natives of this country, 

 who referred them in his " Testacea Britannica" to the former 

 of the two genera just mentioned. Other indigenous species have 

 been discovered in later years, some of which have been de- 

 scribed, and from time to time been made known to the public ; 

 nevertheless, from want of having their characters accurately 

 defined, and still more from not being illustrated by figures suf- 

 ficiently large to convey a correct idea of their relative proportions, 

 it is not always very easy to identify them, nor to determine 

 how far such species are really distinct from one another, or 

 from those before known. It may be added also, that the syno- 

 nyms have been much confused, and the same name has been 

 applied in several instances to more than one species. It is on 

 these grounds that I have been induced to draw up the following 



Vol. IV. Part II. O o 



