164 
ANODONTA — EXTERNAL FEATURES. 
The Isomya are distinguished by tiie approximately eriual size of 
the anterior and posterior adductors (see p. 10, f. 11); the Heteromya 
by the reduction in size and importance of the anterior and the in- 
creased size and importance of the posterior adductor (see p. 10, 
f. T2) ; and the Monom}^, of wliich we have no representative in our 
freshwater tanna, by the anterior adductor being atrophied in tlie 
adult forms, and the posterior adductor only being developed. 
The Isomya te Anodouta cijgnea or anatina, which we may regard 
as tvpifying the Pelecyi)oda, has a more or less oval and laterally 
compressed form, enclosed by amjile lateral mantle lobes which secrete 
upon their external surface a corresi)onding pair of shelly valves, 
united together dor.sally by a somewhat elastic chitinous ligament, 
whose action tends to keep the valves apart ventrally, their closure 
being effected by a pair of powerful adductor muscles passing through 
the body and attached to the inner surface of the valves. 
Fig. — Anodonta anatina^ showing the e.vtended mantle lobes, the anterior position of the 
protrusible foot and the posterior and relative positions of the dorsal orifice, and the branchial and 
anal siphons, as typical of the I^elecypoda. 
a.s. anal or excurrent siphon ; hr.^, branchial or incurrent siphon ; d.o. dorsal orifice ; f. foot ; 
w. mantle margin. 
The External Features of Anodonta may, as in Tlelir, be 
examined according as they belong to the Cephalic, the Pallial, the 
I’edal, or the Visceral regions. 
'Die (V])ha]ic region is degenerate and there are no cephalic eyes. 
'I'lie oval mouth is without prehensile or masticatory organs, Imt is 
surrounded by large and higbly vascular palps or li])S, richly supplied 
with nerves and externally clothed by strong vibratile cilia, whose 
action directs the food-laden currents to the mouth. 
The Pallial region, in onr type, reaches a great develoj)ment, being 
formed by two large, thin, trans})arent, and internally ciliated, leaf-like 
