NAIADES OF MISSOURI 35 



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 vStructures: — vShell 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 co.tated; 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. 



d^ 120 X 72 X 52mm — 0.169 (Platte R., Dixon Falls,) 



9 130 X 80 X 62 " — 0.230 ( " " Garretsburg,) 



9 1 1.5 X 80 X 51 " — 0.215 (Tarkio R., Craig,) 



cf 22 X 20 X 14 " — 0.220 (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 



