NAIADES OF MISSOURI Day] 
closely—even decidously—connected to anal by mantle con- 
nection; inner gills longer and wider—much wider anteriorly, 
inner laminae free from the visceral mass; palpi large, connected 
for about one-third of their length antero-dorsad; except for its 
brownish gills and palpi the soft parts are a soiled white color. 
REPRODUCTIVE STRUCTURES :—All four gills marsupial, ovisacs 
of inner marsupia more extended transversely, giving the white 
conglutinates a leaf-shape; ventral edges pointed; glochidia 
medium size, semi-circular, hinge line long and nearly straight, 
as/long as high, 0.210 mm; conglutinates lanceolate, leaf-like, 
discharged in broken masses, white in color. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell subquadrate, thick, inflated, 
posterior end rather rounded; beaks flattened, sculptured concen- 
trically with two or three nodulous structures at base of flattened 
post-umbonal ridge but disappearing upon the disk; disk with 
but few shallow undulations, sometimes entirely smooth and 
running obliquely to position of post umbonal ridge; slopes of 
post-dorsal ridge rarely costated; epidermis brown or very dark 
horn color. 
INTERNAL STRUCTURES:—Cardinals massive, scars deep, 
umbonal cavity deeply creviced; nacre white with blue, irri- 
descent posterior surface. 
Sex Length Height Diameter Locality. 
Oo 120 xX 72 xX 52mm—o.169 (Platte R., Dixon Falls,) 
OP N30) . x) 80), «x, 62) 7) ——o:230 (ear? ” Garretsburg,) 
OPTI o X60) aXe 5 kn 0-2 15 (Tarkio R., Craig,) 
Ci 22 eX DON Xe Ale p02 20 (Platte R., Agengy Ford,) 
Juvenile shell indicated under last measurement is orbicular 
in outline, has medium inflation and comparatively high beaks; 
however, it does not possess the globose character of peruviana 
and its full beaks are soon lost the older it becomes as determined 
by a good suite. 
MISCELLANEOUS REMARKS:—Amb. rariplicata,.as already 
explained, is the Ohio shell many authors refer to as the plicatus 
of Say, or even as the perplicatus of Conrad. It is to be distin- 
guished from the former since that is the Lake Erie form with 
full beaks; from the latter, however, rariplicata is not so easy 
to distinguish, as its beaks are similar, yet it differs from perplicata 
in being more inflated, with less and shallower plications and with 
