MYTILID^. 261 



anterior oval ; pallial line simple ; ligament external, contained 

 in a very small pit. 



Suborder HETEROMYARIA. 



Frequentl}^ ineqnivalve ; anterior muscular impression very 

 small, posterior impression large. 



{Mytilacea.) 



Family MYTILIDJE. 



Shell equivalve, oval or elongated, closed, umbones anterior, 

 epidermis thick and dark, often filamentose ; ligament internal, 

 submarginal, very long ; hinge edentulous ; outer shell-layer 

 obscurely prismatic-cellular; inner more or less nacreous; pallial 

 line simple ; anterior muscular impression small and narrow, 

 posterior large, obscure. 



Animal marine or fluviatile, attached by a byssus ; mantle- 

 lobes united between the siphonal openings ; gills two on each 

 side, elongated, and united behind to each other and to the 

 mantle, dorsal margins of the outer and innermost laminae free ; 

 foot c^dindrical, grooved. 



The members of this family exhibit a propensity for conceal- 

 ment, frequently spinning a nest of sand and shell-fragments, 

 burrowing in soft substances, or secreting themselves in the 

 burrows of other shells. 



The Mytilidse appear in the palaeozoic strata, and continue in 

 increasing variety and number of species to the present time. 



Subfamily MYTILINjE. 



Shell elongated with subterminal or terminal and pointed beaks ; 

 hinge toothless ; anterior muscular scar small, marginal, posterior 

 scar large and elongated ; pallial line entire, 



Mytilus, Linn., 1158. 



Sea-mussel. 



Distr. — 65 sp. World-wide. Ochotsk, Behring's Sea, Russian 

 Ice-meer; Black Sea, Cape Horn, Cape, New Zealand. Fossil, 

 100 sp. Silurian — ; United States, Europe, South India. 31. 

 smaragdinuH^ Chemn. (cxxviii, 4). 



Shell wedge-shaped, rounded behind, smooth in the typical 

 species; umbones terminal, pointed; hinge-teeth minute or obso- 

 lete; pedal muscular impressions two in each valve, small, simple, 

 close to the adductors. 



Animal with the mantle-margins plain in the anal region, and 

 projecting slightly ; branchial margins fringed ; byssus strong 

 and coarse ; gills nearly equal; palpi long and pointed, free. 



