194 LAKES AND RIVERS. 



their young alive, but they retain them for some time 

 between the mantle and gills. Sometimes they have 

 a byssus or thread proceeding from the foot by which 

 they can suspend themselves. They crawl along on 

 their foot like a leech, and sometimes float in the 

 water. They feed on animalculae, but in winter re- 

 main torpid. The Sphczriidcz are like the marine 

 Kelliadcz ; but the beaks of the shells in the latter 

 are more pointed, and the ligament of the hinge is 

 internal. This family has been carefully studied by 

 the Rev. Leonard Jenyns, and there is much infor- 

 mation about it in the " Transactions of the Cambridge 

 Philosophical Society/ 7 1832. Among French concho- 

 logists M. Bourguignat has produced an elaborate 

 essay on the recent and fossil species of Sphcerium or 

 Cyclas found in France, published in the Memoires de 

 la Societe des Sciences physiques et naturelles de Bordeaux, 

 tome i., 1854. One species only not found in Britain 

 has been noticed by him, the Cyclas solida of Nor- 

 mandy. This, Jeffreys says, forms a link between 

 Sphczrium and Cyrena. M. Bourguignat entitles it 

 generically Cyrenastrum. The Cyrena or Corbicula flu- 

 minalis is found frequently in the upper tertiary beds. 

 The other genus of this family is Pisidium, minutely 

 described by Dr. Baudon in his monograph published 

 at Paris in 1857. Mr. Jeffreys gives the preference 

 to the older name Spharium rather than Cyclas to 

 genus i of this family ; the former name was given 

 by Scopoli in accordance with the spherical shape of 

 the animal. The body in this genus is nearly equi- 

 lateral, and the mantle has a double tube. The shells 

 differ slightly in concavity, and the beaks are near the 



