MYTILIDA. 261 
anterior oval; pallial line simple; ligament external, contained 
in a very small pit. 
SuporvDER HH THROMYARIA. 
Frequently inequivalve; anterior muscular impression very 
small, posterior impression large. 
(Mytilacea.) 
Famity MYTILID A. 
Shell equivalve, oval or elongated, closed, umbones anterior, 
epidermis thick and dark, often filamentose ; Jigament 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 lamine free ; 
foot cylindrical, 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 Mytilidz appear in the paleozoic strata, and continue in 
increasing variety and number of species to the present time. 
SuspraAMity MYTILINA. 
Shell elongated with subterminal or terminal and pointed beaks; 
hinge toothless ; anterior muscularscar small, marginal, posterior 
sear large and elongated ; pallial line entire. 
Myritvs, Linn., 1758. 
Sea-mussel. 
Distr.—b5 sp. World-wide. Ochotsk, Belring’s Sea, Russian 
Ice-meer; Black Sea, Cape Horn, Cape, New Zealand. Fossil, 
100 sp. Silurian—; United States, Europe, South India. J. 
smaragdinus, 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. 
