140 A Guide to the Zoological Collections in the 



but also of many Gastropods, etc. The pearl is formed by 

 the deposition of successive layers of nacre round small 

 foreign bodies that have found their way beneath the mantle. 



We may follow Professor Ray Lankester in dividing 

 the Lipocephala into three Orders, namely, (i) the 

 Isoinya [e.g., the Freshwater Mussel) in which the anterior 

 and posterior adductors are of much the same size; (2) the 

 Heteromya {e.g., the true Mussel of seas and estuaries) 

 in which the anterior adductor is much smaller than the 

 posterior ; and (3) the Monomya {e.g., the Oyster) in which 

 only one muscle — the posterior adductor — is present. 



i. LIPOCEPHALA ISOMYA. 



mszQ X72JI-X77C]. 

 This large Order is divided into two sub-Orders, namely, 

 (i) the Integripallia, in which the siphons, when present, 

 are small and do not lead to an inflection of the pallial line, 

 and (2) the Sinupallia in which large siphons are always 

 present and mark the shell with a " pallial sinus ". 



a. ISOMYA INTEGRIPALLIA. 



[(Hascs 172^-17415]. 

 This sub-Order includes five family-alliances of marine 

 and fiuviatile Bivalves. 



ist Family Arcacea, [Cases 172 A, B, C]. 



In this family the edges of the mantle-lobes are not 

 united below the foot and there are no " siphons ". The 

 shell, which is usually thick and heavy and covered with a 

 tough epidermis, is very often ribbed like a cockle shell. 

 The "hinge-Hne" is very commonly quite straight, and the 

 " hinge-teeth" are usually subdivided into numerous denti- 

 cles of nearly equal size, which are arranged in an equidi- 

 stant series like the teeth of a comb. The great majority 

 of the species are marine, Nucula, Yoldia, and Limopsis 



