142 NATURAL HISTORY. [EAST. ZOOL. 
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 Iridimdce are very like the former, but 
the hinder parts of the mantle are united together, 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) the larger scar of the adductor 
muscle. 
The family of JSIycetopodidce are like the latter, out the 
shell is nearly cylindrical, and widely gaping at each end. 
The foot is very long and cylindrical, and the two anterior 
muscular scars are widely separated, with the smaller one 
before the larger; the lobes of the mantle are free, and 
there are no syphons. 
The family of Trigoniadce have a thick cordate shell 
of a pearly laminar texture, with an external cartilage, 
and the hinge formed of two diverging grooved inter¬ 
locking lateral teeth. The mantle-lobes are free, with a 
large foot, having an acute hooked end, somewhat like 
the cockles, by means of which the animal leaps. 
The family of Arcadce are all marine, and have the 
mantle-lobes free like the former; the foot is com- 
