THE. NAIADES OF MISSOURI 437 
The peculiarity of the shell of this sub-species (as well as that 
of the species) is that the younger the shell the very much thinner. 
It is also more brilliantly tawny and green rayed. 
MISCELLANEOUS REMARKS:—Surely this form of brevicula, 
found by the writer in its type locality, Niangua River, has enough 
peculiar characters to entitle it to a good species, as Simpson had 
first considered it. Its special characteristic is the very wide, deep 
emargination in the female shell at its post-ventral margin. How- 
ever, it is almost identical with its parent species as to its soft parts. 
Its tentacled lamellar-like flap on the mantle edge antero-ventrad 
to the branchial opening is somewhat like that of ventricosa and 
hence might be grouped under the Lamp. luteola group; however, 
the smaller papillae along the posterior end of the flap (or rather 
thickened mantle edge) would class it more as an Eurymia. brittsi 
is to be ‘distinguished from its parent by the greater post-ventral 
sulcus (Fig. 99), which extends in as a rather deep radial furrow 
for a short distance forming the greatest inflation of the shell in 
front of this. It also differs in shell characters from the female 
species (brevicula) in not being so broad posteriorly and not as 
rounded post-dorsad. Dr. Britts collected the originals from 
Niangua River and sent them to Simpson for naming; hence the 
consequent name of this species. Cotypes (now in the hands of 
the writer and illustrated herewith) collected from almost the same 
point in the Niangua show a decided difference from cotypes of 
Call’s brevicula many of which are now in the writer’s collection, 
through the kindness of Mr. B. F. Bush, one of the most active 
students and collectors of Nazad shells now living in this State. 
This sub-species is bradytictic as inferred from the writer’s brief 
breeding record. He has had the good fortune to collect the glo- ° 
chidia of this form for the first time. In all probability this glochi- 
dium is the same as that of Call’s brevicula. However, its breeding 
season seems to be a little different as the writer collected many 
of the species only a day or two later to find them all sterile. 
Sub-Genus Eurynia (Sens. Strict.) Rafinesque. 
1912b—Eurynia (as sub-genus), Ortmann, An. Car. Mus., VIII, p. 338. 
(Type, Unio recta Lamarck.) 
ANIMAL CHARACTERS:—Differs from those of Micromya in 
the structure of its rough mantle edge antero-ventrad to branchial 
opening being more differentiated into a greater number and longer 
