1^2 THE NAIADES OF MISSOURI 



late embryos, brown with glochidia, ventral edge trucated, water 

 canals on either side of undivided ovisacs; glochidia large, spined, 

 spadiform, hinge line undulate, height greater than length (0.350 

 X 0.300mm.). 



SHELL CHARACTERS. 



1 , ^ Exterior Structures: — Shell subrhomboidal, inflated — 



^ ^ extremely so along the sharply angled, post-umbonal ridge; post 



dorsal ridge low with broad gentle slopes finely costated; disk 

 [^ smooth; beaks long full, sculptured with heavy concentric bars, 



the later ones undulated low almost to disk; epidermis smooth, 



polished, with spotted, greenish rays from anterior portion of shell 



to posterior ridge. 



Interior Structures: — Cardinals single in each valve, inter- 



dentum displaced by saddle-shaped tooth in left valve; laterals 

 kV^' reduced to rounded edges; muscle cicatrices faint; shell cavity 



deep; nacre whitish to pearl blue and pinkish. 



Sex Length Width Diameter Locality 



9 75 X 40 X 34 mm — (Gasconade, R., Gascondy, Mo.) 

 cf 60 X 31 X 24 " — ( " " " " ) 



9 66 X 35 X 24 " — (Jack's Fork, Shannon, Co. Mo.) 

 9 37-5 X 21.5 X 11.5 " — ( " " " " " ) 



The juvenile of this last measurement presents the same 

 sculpturing as in the juvenile calceolus except that the bars are 

 somewhat more elongated in niarginata and are really lower and 

 coarser extending down well on the disk. 



Miscellaneous Remarks: — It has been well known by 

 Pilsbry and Fox that this western shell is not the Alasmidonta 

 truncata Wright, mentioned by vSimpson. This species is easily 

 recognized by its post inflated shell making the post - dorsal portion 

 almost truncated and also by its extremely coarse beak sculpture. 

 Its very long narrow foot extension is a notable physiological 

 character. It delights in sandy, pebbly situations. The muddy 

 waters of North Missouri is not conducive to its distribution 

 there and is very rare in the Osage basin; however, it is rather a 

 common shell in the Gasconade where it reaches its greatest per- 

 fection and is commonly distributed throughout the mountain 

 streams of the south. Occasionally niarginata is found in the Miss- 

 issippi north of the Missouri River. The author has records of 



