ho 
NAIADES OF MISSOURI 99 
anterior end; outer lamina of outer gill slightly free posteriorly 
from mantle, all four gills marsupial; palpi, large, comparatively 
broad, hangs low, united two-thirds of way toward base; color 
of soft parts soiled white, mantle edge blackish chiefly at the 
branchial openings. 
REPRODUCTIVE STRUCTURES:—No gravid females found so 
far.' The gills of several specimens from the Mississippi River 
presented no variations of structure; hence this peculiar oblique 
arrangement of septa may not be a sex distinction. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Elongate-elliptical, arcuate in old 
specimens, not sexually dimorphic; rounded before, usually more 
pointed behind; beaks small, low, sculptured by a few coarse 
concentric bars; lines of growth rough, coarse; epidermis black, 
shell moderately thick anteriorly, but very thin posteriorly, 
being disposed to crack easily upon exposure to air. 
INTERNAL STRUCTURES:—Cardinals conical, single in both 
valves, rudimentary to lacking in left; laterals low, single in right, 
inclined to double posteriorly in left; scars well impressed anteri- 
orly—especially the one taking the position of anterior cardinals 
in most other Nazad shells; beak cavities shallow; nacre bluish 
with a slight tint of salmon in umbonal cavity; no vein marking 
as in most Unioninae. 
ORAS Xora Ar5 ak 25 TT (Osage R., Bagnell, Mo.) 
Cen eX d's KU 2705 1 Gi tiwiae woagradaLMvio!) 
rE 122) Lax gird, x 26mm (Miss. R., Louisiana, Mo.) 
rot OOn xen a0 xX 20.5mm (Miss. R., Louisiana, Mo.) 
MISCELLANEOUS REMARKS:—C. monodonta is most typical in 
the Mississippi above the mouth of the Missouri. Bryant Walker 
records it from ‘Tennessee to Ohio, thence Northwestward to 
Nebraska. From the fact that the author found this primitive 
species at several points in the Osage and Gasconade Rivers, its 
known distribution 1s now carried farther south and west of the Miss- 
issippr River, than recorded before. The existence of this, as well as 
other primitive forms of the Nazades, also in the Cumberland- 
* Dr. A. D. Howard, (Scientific Assistant, U. S. Biological Station, 
Fairport, Iowa), has however recently discovered that this species bears 
unusually small glochidia and has the peculiar habit of bearing two broods 
in a season (Nautilus, XXIX, p. 6, May, 1915.) 
