Invertebrate Gallery of the Indian Museum. 145 



The typical Mussels [Myttlidce] are marine and are 

 found in all parts of the world, living together in vast 

 beds. 



ill. LIPOCEPHALA MONOMYA. 

 [€as£s 178^—179(1]. 



This Order includes the Oysters, Scallops, Pearl- 

 oysters, etc., in which in the adult the anterior adductor 

 muscle is absent (see the prepared shells of Oyster and 

 Scallop in Case 172A, and the dissected spirit specimens 

 of Amussium and Spondyliis above Case 179). 



1st Family Aviculacea, [Case 178 B, C]. 



The lobes of the mantle are separate throughout and 

 fringed, and there are no siphons or siphonal folds : the 

 foot is small and secretes a byssus for permanent attach- 

 ment. The shell is generally covered on the outside with 

 a thick, fibrous, often vallanced epidermis, which is com- 

 monly of a dark green or black colour. The internal 

 "mother-of-pearl" lining, though often of surpassing 

 lustre, is always limited in extent, sometimes not occupy- 

 ing as much as a fourth of the whole internal area. Hinge- 

 teeth are usually absent. The family though small is 

 widely distributed : it includes many large forms, among 

 them the famous Pearl-Oyster of the Indo-Pacific. 



2nd Family Ostracea, [Cases 179 A, B, C,]. 



In this family, which includes the Scallops {Pecten), 

 the Thorny Oysters [Spondylus), and the true Oysters 

 {Ostrea), the edges of the mantle, which are quite free 

 throughout, are fringed with tentacles, and are often fur- 

 nished with a row of highly developed eyes (see the dis- 

 sected spirit specimens of Amussium and Spondylus 

 above Case 179). There are no siphons. Hinge-teeth 

 are usually absent, but are well developed in Spondylus. 

 The Pectens, though able to secrete a byssus for (tem- 

 porary) attachment, are able to progress with great activity 



