NAIADES OF MISSOURI 25 



and more typical shell of the main species. This subspecies, 

 however, is more like the type than the trigonoid?s of Frierson, 

 the latter being more nearly the flava type and hence might be 

 well named "F. undata flava (Raf.)" 



Fusconaia undata trigonoides Frierson, MS. 



("Big Pigtoe.") 



PI. XV— Figs. 30 A—D; PL IV, Figs, ga, and gh. 



1913 — Fusconaia undata trigonoides Frierson MS. — Personal Letter, 

 February 22, 1913. 



Animal Character: — Soft parts indentical with species. 

 Glochidium measures 0.180 x 0.165mm; conglutinates white in 

 glochidial and late embroyonic stages, pink in earlier stages. 



SHELL CHARACTERS. 



External Structures: — Shell elongate trigonal, thick, 

 heavy, very large for the genus; disk without sculpturing except 

 that rest-lines are somewhat furrowed giving an undulate appear- 

 ance to growth lines; beaks comparatively low and not placed 

 so much anteriorly, sculptured with concentric bars breaking into 

 two or three nodules at base of post-umbonal ridge; post-dorsal 

 part of shell, bowed, slightly biangulated behind ; epidermis 

 cloth-like, reddish brown to horse bay color, rayed in young. 



Internal Structures: — Identical with those of the species 

 and other intergrading forms, except that it may vary from a 

 white to pink nacre. 



Length Width Diameter Location 



93 X 66 X 42 (Platte R., Dixon Falls, Mo.) 



70 X 50 X 38 (Osage R., Schell City, Mo.) 



50 X 40 X 24 (Osage R., Warsaw, Mo.) 



28 X 20 X 14 (Platte R., Gari'etsburg, Mo.) 



The juvenile indicated by the measurement under the last 

 number was found lying on its side in the shallow water of a riffle. 

 It is different from the mature shell in its dark yellow epidermis 

 marked all over by green rays, also by more prominent umbonal 

 ridge, more centrally placed and more distinctly sculptured beaks. 

 The soft parts are all of a bright orange color except the bluish 

 visceral mass. 



This specimen and also the one under the first measurement 

 of the above list were kindly indentified by Dr. Ortmann as F 



