NAIADES Olf MISSOURI I7 



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 Naiad shells; beak cavities shallow; nacre bluish 

 with a slight tint of salmon in umbonal cavity; no vein marking 

 as in most Unioninae. 



9 115 X 44.5 X 25mm (Osage R., Bagnell, Mo.) 



cf 127 X 45 X 27.5mm ( " " Sagrada, Mo.) 



9 122 X 47 X 26mm (Miss. R., Louisiana, Mo.) 



cf 96 X 40 X 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 is now carried farther south and west of the Miss- 

 issippi River, than recorded before. The existence of this, as well as 

 other primitive forms of* the Naiades, 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.), 



