, 
THE NAIADES OF MISSOURI 273 
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. 
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 
reduced to rounded edges; muscle cicatrices faint; shell cavity 
deep; nacre whitish to pearl blue and pinkish. 
Sex Length Width Diameter ; Locality 
9 75 xX 40 x 34 mm—(Gasconade, R., aaa Mo.) 
GOs BOX Si x. 2a 5! a a ) 
OOO). S635 ox 24° 7 —=(Jack’s Dae Set Se Mo.) 
eM crs LG ke ah tak (ht zt ) 
The juvenile of this last measurement presents the same 
sculpturing as in the juvenile calceolus except that the bars are 
somewhat more elongated in marginata 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 Simpson. 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 marginata is found in the Miss- 
issippi north of the Missouri River. The author has records of 
its breeding season for August through to December, a sufficient 
record to know that it is a long period breeder (bradytictic.) 
