115 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
The family of Crassinidce only differ from the former 
in the teeth being triangular and diverging like the Vene - 
ridce , and they are covered with a thick brown periostraca; 
the hinder lateral tooth of the left valve is double. 
The family of Crassatellidce are nearly similar, but 
they are at once known by the cartilage being placed in 
an internal triangular pit near the cardinal teeth. 
The family of Isocardiadce have the very oblique cardinal 
teeth and the linear external cartilage of Cardita , but the 
hinder lateral teeth of the right valve are double. 
The family of Lucinidce have an orbicular white shell, 
with none or only a few very small cardinal teeth, and the 
mantle-lobes are more or less united together beneath, 
leaving only a hole for the elongate strap-shaped foot. 
They have a single, conical, contractile anal syphon over 
the aperture for the entrance of the water ; the front ad¬ 
ductor muscle is very long and high, and the inner disk 
of the shell is opake and rugose. In general the carti¬ 
lage is marginal under the ligament, but in Loripes it is 
internal in an elongated pit. 
The family of Unionidce, or of Pond Muscles, are, like 
the Etherice , only found in fresh water ; they are covered 
with a hard olive periostraca, under which the shell is 
hard and beautifully pearly. The mantle-lobes are free 
beneath ; the cartilage is always marginal, and the hinge 
presents several modifications, but is always destitute of 
any cardinal teeth ; in Anodon it has no teeth at all; in 
Unio , Damans , &c., it has lateral teeth of different 
degrees of developement and form, so that they some¬ 
times resemble cardinal ones. 
The family of Iridinidcz are very like the former, but 
the hinder parts of the mantle are united and produced 
into two short, unequal, separate syphons. The shell 
is solid, and the hinder part of the submarginal scar is 
more or less indexed. In Iridina and Leila the hinge 
edge is smooth, like Anodon , and the latter has a sharp 
syphonal inflexion. The Pleiodon has a series of trans¬ 
verse teeth, like Area , but they are irregular; and Hyria 
(and Castalia ) have compressed crest-like lateral teeth, as 
in Unio . The two latter genera may be distinguished 
from Unio , by having the smaller anterior scar placed 
over (and not under, as in Unio ) the larger scar of the 
adductor muscle. 
