SPHiERIIDiE. 6 



KelUadce, and the beaks of their shell are much more 

 acute. This curious and distinct group of freshwater 

 bivalves has been carefully investigated by our country- 

 man^ the Rev. Leonard Jenyns ; and his monograph on 

 the British species of Cyclas and Pisidium, which was 

 published in the Transactions of the Cambridge Philo- 

 sophical Society for 1832, is full of valuable and inter- 

 esting information. Since that time the labours of natu- 

 ralists have been divided even in this humble and com- 

 paratively obscure study. Several French conchologists, 

 especially MM. Normand and Gassies, have separately 

 devoted themselves to a critical examination of their 

 native species of the above genera; and lately M. Bour- 

 guignat has favoured the scientific world with an elabo- 

 rate essay on the recent and fossil species of Sphcerium 

 (or Cyclas) which have been found in France. This 

 essay was published in the ^ Memoires de la Societe des 

 Sciences physiques et naturelles de Bordeaux/ tome i. 

 1854. The only recent species described or noticed by 

 him, which is not also found in this country, is the 

 Cyclas solida of Normand. It appears to form an inter- 

 mediate link between Sphcerium and Cyi^ena; and M. 

 Bourguignat has separated it from the former under the 

 generic name of Cyrenasti'um. I mention this in con- 

 sequence of the Cyre7ia (or Co7'bicula) fluminalis oc- 

 curring so frequently as a fossil in our upper tertiary 

 beds, and in the hope that the Cyrenastrum solidum may 

 also turn up in the same deposits, and lead to an eluci- 

 dation of the question how the limits of the true Cyrena, 

 in its living state, have become so restricted since the 

 glacial epoch. The only other genus of this family [Pisi- 

 dium) has lately had an equal amount of laborious atten- 

 tion bestowed on it by an eminent member of the French 

 corps of conchologists. The ' Essai monographique sur 



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