142 A^Guide to the Zoological Collections in the 



[Galeomma) they are united in front and prolonged behind 

 into two siphons ; and in other cases {Lucina) they are in 

 an intermediate condition, being separate in front and 

 united behind, somewhat as in the Freshwater Mussels, to 

 form siphonal pockets. In Kellia the mantle-lobes meet 

 and are produced in front^ to form a respiratory tube. 

 The Lucinacea are marine. 



5th Family Cyprinacea, [Cases 174 A, B]. 



This family includes the Clams and Cockles. The con- 

 dition of the mantle here also varies : in all cases the lobes 

 are united behind to form distinct but short siphons, the 

 free edges of which are usually fringed with tentacles ; but 

 whereas in the Cockles they are separate in front, in the 

 Clams they are united, thus closing the mantle-cavity. 

 The Cockles are of universal distribution and occur both in 

 the sea and in freshwater: the Clams are entirely tropical 

 and marine. The Cockles are freely locomotive, using 

 their curved foot for leaping : the Clams, on the other hand, 

 are fixed, either being attached by a strong byssus, like 

 the Tridacnas, and deeply wedged into growing coral-rock; 

 or, as the Oyster-like Chamas, being firmly adherent to 

 the rock by the surface of one valve. 



Some species of Tridacna reach an enormous size : a 

 specimen nearly four feet long, and more than two men's 

 load in weight, is placed below Case 174. 



/3- ISOMYA SINUPALLIA. 



[cias.c0 i74(!r-i77ar]. 



This sub-Order includes three family-alliances of chiefly 

 marine Bivalves. 



ist Family Veneracea, [Cases 174C— 176B]. 



In this family, which is one of the largest of the whole 

 Class, the lobes of the mantle, though widely open in front" 

 for the passage of the foot, are united below and behind, 

 where they are prolonged out to form two tubular siphons 



