206 BRITISH MOLLUSKS. 



happens that the only member of this Order inhabiting fresh water 

 in Europe, belongs to a family plentiful in marine species, in which 

 the characteristic muscles of the Order undergoes a change. The 

 adductor muscle becomes, in the Mytilacea, a compound muscle, 

 and in the anterior extremity of the shell of some of them there are 

 two small auxiliary muscles. 



The only British freshwater species of Unimusculosa belongs to 

 the family. 



1. Mytilacea. Animal bearing an oblong fan-shaped shell, to 

 which it is affixed posteriorly by a pair of strong compound 

 adductor muscles. Beneath the umbo in each valve, there is 

 sometimes a small auxiliary muscle affixed to a cross septum. 

 Adhering to foreign bodies by a byssus of firm tendinous 

 threads, proceeding from a groove in the foot. 



Family I. MYTILACEA. 



Animal bearing an oblong fan-shaped shell, to ivhich it is affixed ante- 

 riorly by a pair of strong compound adductor muscles. Beneath the umbo in 

 each valve, there is sometimes a small auxiliary muscle affixed to a cross 

 septum. Adhering to foreign bodies by a byssus of firm tendinous threads, 

 proceeding from a groove in the foot. 



The Mytilacea are a family of about a hundred and eighty species, 

 of which a hundred and seventy, belonging to the genera Mytilus, 

 Modiola, Crenella, and Lithodomus, inhabit the sea, and the re- 

 maining ten are inhabitants of brackish and fresh water. Of two of 

 these, the habitats are not known. Six are natives of Central 

 America and the Southern United States, one is a native of Senegal, 

 and one of Europe. The freshwater Mytilacea differ from those 

 of marine habit, and from the rest of the unimusculose Ac&phala, in 

 having the mantle closed. 



The main adductor muscle of this family adheres rather near to 

 the posterior edge of the shell, and the cicatrix indicative of its 

 place of attachment is of a duplicated oblong form, showing it to be 

 of a compound structure, but it is not divided into bundles of fibres 

 in Dreissena as in Mytilus. Beneath the umbo in each valve, there 

 is a small shelly septum, and affixed to these is a small auxiliary 

 transverse muscle. 



