124 THE AMERICAN MIDLAND NATURALIST 
edge; conglutinates, sole-shaped, brown, rather solid; glochidia 
large, ventral margin obliquely rounded, hinge line long. 
SHELL CHARACTERS. 
Shell latge, ponderous, broadly rhomboid, moderately inflated, 
post-dorsal ridge alated, sculptured with regular upcurved undu- 
lations; post-umbonal ridge broken with coarse plications running 
more or less parallel with it; beaks rather low, sculptured with 
coarse double looped-corrugations which extend out as nodules at 
base of post-ridge and as zigzag ridges all over umbonal region to 
upper part of disk; epidermis black; cardinals heavy; laterals 
long and straight; interdentum short; beak cavities narrowly 
deep; scars very deeply impressed—especially anterior retractor 
cicatrix; nacre white to pink. 
MISCELLANEOUS REMARKS:—Because of the peculiarities of 
heros (Say) as to its animal and shell characters, as well as to its 
uniqueness of breeding season it 1s thought by the author in con- 
ference with other students, that this species of Say, very well deserves 
rank as the type of a new genus. Although the author has not 
examined the animal of boykiniana Lea, triumphans Wright, etc., 
yet, from shell characters, these allied forms would naturally 
fall under this new genus, Megalonazas. In all probability crasst- 
dens (Lamarck) |=trapezoides (I,ea)], which has been grouped 
very near heros (Say), may also deserve a special compartment, 
according to the recent opinion of Mr. Frierson, who has made 
special study of this species abundant beds of which are very 
accessible to him; hence because of the difference of shell characters 
of crassidens from that of heros (or from any other Nazad shell 
in the possession of a ‘“‘ ventral scar’’ as pointed out by Mr. Frierson) 
this species of Lamarck is not grouped there. Besides crassidens 
is not found in Missouri, neither is boykiniana, triumphans and 
other conchologically allied forms of M. heros and thus the new 
genus will safely stand out for this State with its type, (heros Say), 
as the lone representative. Lartosta (Raf.) might be the available 
name for our new genus, if crasstdens could be found to be congen- 
eric with heros, since Rafinesque erected his genus for this species 
which he termed ponderosus, but which Mr. Walker, through his 
close study of Lamarckan types, says is Lamarck’s crassidens 
that ante-dates Lea’s trapezoides as well as Rafinesque’s type. 
From the fact that Crenodonta (Schliiter) falls into the synonomy 
