THE NAIADES OF MISSOURI 393 
and very much so ventrad from original edge of sterile marsupium, 
ventral tips of ovisacs teat-like, not colored; conglutinates white, 
discharged in broken disintegrated masses; glochidium ax-head 
shape or celtiform, small with spine-like structures measuring 
0.100 xX 0.155mm; branchial mantle edge thickened, lamellar, but 
without palpilae. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell elliptical compressed, thin, 
bialated, post-ala being drawn near to the beaks in definite lobes 
of growth, sometimes curved laterally; disk without any sculpture; 
post-umbonal ridge absent, female shell swollen post-ventrad; 
epidermis brown-glistening horn-color with faint rays and areas 
of indigo blue especially on post wing; beaks low, suppressed 
sculptured with a few fine concentric lines and a row of three small 
tubercles on line with a post-ridge. 
INTERNAL STRUCTURES:—Cardinals thin, erect, double in 
right, single in left valve, laterals rather reduced; scars somewhat 
faintly impressed; beak and branchial cavities rather shallow; 
nacre solid purple. 
Sex Length Height Diameter Locality 
S50 eos) xXx) 42 tm (102 R., St. Joseph) 
Ol MALS) OX Ole ke aeXopaohon (Mud Lake, Kenmoor) 
ORIG Eno On lee au TnI (Platte, R., Agency Ford) 
OPA AX’ 23) 1 ox hr atin (Lake Contrary, St. Joseph) 
The last measurement of a juvenile—the youngest and smallest 
Natad shell ever found gravid by the writer. Its glochidia were 
normal. Many of these juveniles are in the writer’s cabinet, 
having been collected in ‘‘nests’’ from L. Contrary for the most 
part. The shells are like those of ground glass in color and trans- 
luscent both externally and internally. Beaks are rather apicu- 
lated and marked by rather coarse concentric ridges with three 
teat-like tubercles arranged in a row on line with post-ridge, resemb- 
ling juvenile beak sculpture of Lasmonos. 
MISCELLANEOUS REMARKS:—This species may represent the 
critical transition period from the primitive to the actual modern 
forms. Its glochidium is not a true Proptera form in not possessing 
typical spines at the ventral corners of its valves. Coker and 
Surber (1911, pp. 179-182) have pointed out its metamorphosis in 
the parasitic life as eccentric in that the glochidium remains 
