46 THE CHICAGO ACADEMY OF SCIENCES. 
cles; a more or less convoluted intestinal canal, with its oral 
and anal extremities at opposite ends of the body; a stomach; 
paired nephridia, connected with the pericardium, and discharg- 
ing independently of the rectum; reproducing without copula- 
tion, by eggs and spermatozoa; moncecious or dicecious; devel- 
opment external to the ovary; the post-larval young protected 
by a prodissoconch, and sometimes exhibiting a nepionic stage; 
with a distribution in geological time from the Cambrian to the 
present day.” (DALL.*) 
Fig. 9A shows the position of a fresh-water pelecypod in 
life, while ploughing its way through sandy or muddy bottoms. 
ORDER PRIONODESMACEA. 
«‘ Pelecypods having the lobes of the mantle generally sep- 
arated, or, when caught together, with imperfectly developed 
siphons; the soft parts in general, diversely specialized for par- 
ticular environments; the shell structure nacreous and prismatic, 
rarely porcellanous; the dorsal area amphidetic or obscure, 
rarely divided into lunule and escutcheon, and when so divided 
having an amphidetic ligament; ligament variable, rarely opistho- 
detic; armature of the hinge characterized by a repetition of 
similar teeth upon the hinge-line, or by amorphous schizodont 
dentition; habits active, sessile, or nestling, not burrowing; 
moncecious or dicecious.’”’ (DALL.*) 
Superfamily Naiadacea. 
‘Shell of varied form, normally equivalve, inequilateral and 
dimyarian; rarely alate; shell substance nacreous and prismatic, 
with a conspicuous epidermis; area obscure or amphidetic; lig- 
ament parivincular, usually opisthodetic and external; ventricle 
embracing the rectum, with anterior and posterior aortas; gills 
reticulate, with direct and reflected laminz, one or both pairs 
frequently utilized as a marsupium; pleural ganglia fused with 
the cerebral; otocyst impervious; pallial lobes usually free, 
except for an anal siphon; the pallial line simple; foot nor- 
mally long, compressed, keeled, the byssus obsolete; anal end 
of rectum adherent; young usually with a distinct nepionic 
stage; station usually fluviatile.” (Da.ty.) 
*Trans, Wagner Free Institute of Science, Vol. III., Part 3, p. 511, 1895. 
+ Contr. to Tertiary Fauna of Florida, etc., Trans. Wagn. Free. Inst., Vol. III., Part 1, 1890 
