126 MYTILIDJE. 



shaped ; the central area is depressed ; and the beaks 

 are placed at a greater distance from the anterior 

 margin. Mr. Dennis says that it is frequently imbedded 

 in a very snug little nest on sea- weeds, but never among 

 their roots, where M. discors takes up its abode and 

 occurs in great numbers. My largest specimen is half 

 an inch long; but I believe it is usually very much 

 smaller, and that in cubical contents it is not a tenth of 

 the last species. 



3. M. dis'cors*, Linne. 



Mytilus discors, Linn. Syst. Nat. p. 1159. Crenella discors, F. &H. ii. 

 p. 195, pi. xlv. f. 5, 6 ; and pi. xlviii. f. 5. 



Body yellowish-white, with a few scattered minute specks 

 of flake-white : incurrent tube formed by a wide protube- 

 rant slit of the mantle in front for the admission of food and 

 water: excurrent tube conical, and projecting considerably 

 beyond the shell at the posterior or broader end : foot strap- 

 shaped, capable of being extended more than twice the 

 length of the shell, and used (like that of Mytilus edulis) for 

 crawling. 



Shell oval, much broader than either of the other species 

 above described, somewhat compressed, rather thin, usually 

 not glossy : sculpture, 10-12 remote ribs on the anterior side, 

 and 30-40 close-set ribs on the other side ; middle area de- 

 pressed and smooth ; transverse stria? very minute and fine : 

 colour yellowish-brown : epidermis rather thick, light green : 

 margins rounded on all sides, except the ventral, which is 

 nearly straight : beaks small, rather prominent, incurved and 

 diverging, placed near the anterior margin : byssal sinus, 

 ligament, and hinge-line as in the other species : hinge-plate 

 strengthened by an internal rib, and obliquely and strongly 

 toothed ; ligamental groove narrow : hinge slightly reflected 

 and thickened, crenulated: inside highly nacreous, purplish- 

 yellow or liver- colour, with the anterior and posterior edges 

 finely notched : scars rather distinct. L. 0-5. B. 0-35. 



Var. 1. angustior. Shell smaller, narrower, and more glossy. 



* Disagreeing. 



