FAMILY 8. CONCHACEA (FLUVIATILIA). 85 



Family 8. CONCHACEA. 



Testa orbicularis, vel transversa ; dentibus cardinalibus in valva altera 

 tribus aut pluribus, in altera tribus aut minoribus ; lateralibus qui- 

 busdam in utraque valva, interdum nullis. 



The Conchacea constitute a numerous family, distinguished above 

 most of the Bimuscular Tropiopoda by the elegance and beauty of their 

 shells. They pass through a considerable variety of form, and the gra- 

 dual modification throughout the series from one species to another has 

 given rise to much speculation on their distribution into genera. In the 

 subdivision of this family, the whole of which is included in the genus 

 Venus of Linnaeus, the first object has been to separate the fluviatile 

 from the marine. 



CONCHACEA FLUVIATILIA. 



Testa epidermide crassa induta ; cardine dentibus lateralibus instructo. 



The fluviatile Conchacea are not very numerous in species ; they in- 

 habit rivers, lakes, ditches, fens, etc., and may be recognised as having 

 their shell covered with a thick dark epidermis, generally much eroded 

 towards the umbones. The hinge is characterized as having two or more 

 distinct lateral teeth, which that of the marine Conchacea has not. 



The Conchacea fluviatilia are divided into three genera, as follows : 



Cyclas. 

 Cyrena. 



GALATHiEA. 



