MYTILUS. 103 



behind, so as to form a separate opening for excretal 

 purposes, as well as in the shell increasing excentrically, 

 and not being scaly but provided with an epidermis, 

 and in its beaks being incurved instead of straight. 

 Although all mussels spin a byssus, and are commonly 

 attached by this mode, some of them have a consider- 

 able power of locomotion, dropping their anchors and 

 casting new ones at will. The composition of their 

 shells, according to Mr. Sorby, is calcite, and ought 

 therefore to be durable; but I have generally found 

 them to be much decomposed in the newer tertiaries. 

 The anterior adductor muscle is very small and placed 

 immediately within the beaks of the shell. 



Genus I. MY'TILUS * Linne. PL III. f. 2. 



Body subcorneal : mantle widely open in front : foot strap- 

 shaped. 



Shell oblong or oval : hinge often furnished with small 

 tubercular teeth. 



The Linnean characters of Mytilus were very various, 

 and comprised freshwater as well as marine shells. 

 Bruguiere was the first to restrict and define the genus 

 as it is now recognized. Lamarck separated from it 

 certain species to form his genus Mocliola ; but I believe 

 the opinion that they ought to be reunited is held by 

 most authors who have bestowed much attention on the 

 classification of the Mollusca. The only indications 

 which Lamarck has given to distinguish Modiola from 

 Mytilus are that the beaks of the shell in the first of 

 these genera are not quite terminal, and that the ani- 

 mal is seldom fixed by a byssus. But on neither ground 

 can the genera be separated. Any one who has exa- 



* From [xvtiXos, a name given by the Greeks to this sort of shell-fish. 



