266 MYTILIDA. 
adductor supported on a shelf within the beak; pedal impres- 
sion single, posterior. 
Animal with the mantle closed; byssal orifice small; and 
siphon very small, conical, plain, branchial prominent, fringed 
inside; palpi small, triangular; foot-muscles short and thick, 
close in front of the posterior adductor. 
D. polymorpha (exxviii, 100; exxix, 24) is a native of the 
Aralo-Caspian rivers ; in 1824 it was observed by Mr. J. Sowerby 
in the Surrey docks, to which it appears to have been brought 
with foreign timber, in the holds of vessels. It has since spread 
into the canals, docks, and rivers of many parts of England, 
France and Belgium, and has been noticed in the iron water-pipes 
of London, incrusted with a ferruginous deposit. 
MYTILUPSIS, Conrad, 1857. (Praxis, H. and A. Ad., 1857.) 
Shell with a lamina on the hinge-shelf or septum. WD. Sallet, 
Recluz (cxxix, 22). D. leucophzata, Conrad. Brackish waters 
of Chesapeake Bay, on oysters. 
DREISSENOMYA, Fuchs. Septum transformed into a regular, 
large, anterior muscular scar, pallial line with a deep posterior 
sinus. D. Schrockingert, Fuchs. Upper Tertiary; Hungary. 
SEPTIFER. Recluz, 1848. 
Distr.—Warm seas. Fossil; Jurassic and Cretaceous. S. 
Heberti, Desh. (cxxix, 28). 
Shell equivalve, very inequilateral; ventral margin subconcave 
and cut out for the passage of the byssus; beaks subterminal, 
curved ; hinge without teeth, furnished with a lamellar septum ; 
ligamental pits linear, marginal, dorsal, anterior, with a white, 
nearly spongy margin within; muscular impressions superficial, 
the anterior small, rounded, the posterior large, subdorsal, 
uniform. 
Animal marine, byssiferous. 
Myatina, Koninck, 1842. 
Distr.—Fossil, 6 sp. Silur., Carb.—Permian; Europe. 7. 
lamellosa, Koninck (cxxix, 25). 
Shell equivalve, mytiliform; beaks nearly terminal, septiferous 
internally ; hinge-margin thickened, flat, with several longitu- 
dinal cartilage-grooves ; muscular impressions two; pallial line 
simple. The ligamental area resembles that of Arca obliquata, 
Chemn. 
ANTHRACOPTERA, Salter, 1863. 
Syn.—Naiadites, Dawson, 1855 (part). 
Etym.— Anthrax, coal, and pteron, a wing. 
Distr.—Fossil, 10 sp. Carboniferous; Great Britain, West- 
phalia, Nova Scotia, United States. 
