162 



MYTILID^. 



The true Mussels are mostly inhabitants of the sea, a 

 few only dwelling- in fresh water. They have more or less 

 elongated and very inequilateral shells, always, however, 

 equivalve. The hinge is of a very simple structure, gene- 

 rally devoid of teeth ; the ligament is linear. The surface 

 of the shell is smooth, or rugose, or obliquely striated and 

 sulcated, and is often covered by an epidermis. The animal 

 has always two adductor muscles and a narrow strap-shaped 

 foot, furnished with a byssal groove. It can spin a very 

 strong and copious byssus, though, as in the case of the 

 Lithodomi^ this is in some species only done when it is 

 young. The mantle has its edges variously united in some 

 genera, and apart in others, so that some forms have distinct 

 siphonal tubes and others none. Usually the anal tube is 

 complete, and the branchial only rudimentary. 



The structure of the shell in the Mytilida has been sub- 

 mitted to microscopic examination by Dr. Carpenter, with 

 interesting results. He finds the shells of Mytilus^ Modiola 

 and LitJiodomus all to possess a periostracum of horny 

 membrane in which no distinct structure could be detected. 

 Between it and the shells a thin layer of minute cells may 

 frequently be observed. The shell itself presents two layers, 

 distinguishable by their colour ; the internal one is often 

 iridescent and always nacreous. Neither layer in the two 

 former genera presents very distinct organic structure, but 



