478 MOLLUSCA. 
Récluz2 says that the animal pierces the rocks; it is more probable, however, that 
it penetrates only the holes that are already there, and adapts its further growth to the 
surrounding space. 
A species from the Rio Hacha, Colombia (in the Berlin Museum), differs from 
D. sallei in being more elongate and a little more flattened, with the summits less bent 
down, the colour paler, and the appendage of the septum usually more excavated. 
D. domingensis, Récluz, from Haiti and Venezuela, is of smaller size and more convex, 
with the summits also less bent down and the appendage of the septum more excavated. 
There are, however, so many individual variations in a set of shells collected at the 
same locality, that all these differences hold good only for the majority of specimens 
from any one place. 
Fam. UNIONIDZ. 
Freshwater Bivalves of moderate or large size, with distinct greenish, brown, or 
blackish periostracum (epidermis) ; inside more or less pearly, with two nearly equal 
impressions of the adductor muscles, one before and the other behind, and some smaller 
impressions of the pedal muscles. Borders of the mantle for the greater part disunited, 
united in the posterior dorsal margin only: apparently two round holes for the entrance 
and exit of water; but only one of them, the upper, really closed all round, the other, 
lower, continuous with the general cleft of the mantle and fringed with cirri. Shells 
generally longer than high, with or without teeth in the hinge. 
Found on all continents, even in Australia, and also in large or moderately large 
islands, such as Great Britain, Cuba, Sumatra, Java, Borneo, and New Guinea; but 
wanting on smaller islands, even in Jamaica, Haiti, and Celebes. 
UNIO. 
Unio, Retzius, Dissert. Hist.-Nat. sistens nova Test. gen. resp. Philipsson, p. 16 (1788) (part.) ; 
Bruguiére (1792), Schumacher (1817) (as at present restricted), &c. 
Shell solid, usually oblong or elongated, shorter and rounder before the summits, 
lengthened and often more or less pointed behind them; summits, if not worn, 
sculptured. Hinge toothed: before the summits one or two teeth (the anterior one 
much the smaller when present) in the right, two nearly equal in the left valve, 
triangular or compressed, lamelliform, mostly furrowed and crenulated (called cardinal 
teeth by most authors and anterior lateral teeth by Crosse and Fischer); behind the 
summits one elongated, compressed, lamelliform tooth reaching behind the hinder end 
of the ligament in the right valve, two such teeth in the left valve. 
female shells sometimes more convex. 
For the history of the genus and the anatomy, see Fischer and Crosse, Miss. Scient. 
Mex., Mollusques, ii. pp. 538 e¢ seq. 
Geographical distribution :—On all continents and some islands; northwards it 
extends to the central] part of Sweden (province Dalarne) and to Canada, southwards to 
Sexes separate ; 
