134 THE NAUTILUS. 
epidermis, an obsolete umbonial ridge and deep, compressed beak- 
cavities. It differs in much the same manner from U. Lesleyi, which 
has greater shell cavity. Compared with U. abacus Hald., it is pro- 
portionally much wider, deeper and with more compressed beak- 
cavities, and has not the checker-board markings of that species. 
Old specimens resemble U. Pi/sbryi Marsh, interiorly, but without 
alation or undulations. In old specimens the base becomes emargin- 
ate, and there is more or less biangulation behind, and the teeth be- 
come almost obsolete, and of a dull smoky tinge. Thirty specimens 
were received. 
Unio Friersoni sp. nov. Pl. ITI, figs. right and left. 
Shell smooth, triangular, inflated, nearly equilateral, obtusely 
pointed behind ; sides swollen not constricted, rayless or obscurely 
radiate when very young; substance of the shell uniformly rather 
thick and solid. Beaks angular to the tips and with three or four 
close undulations which form parallel V-shaped ridges with the 
angle pointing down the umbonial angle, and the long straight arms 
of which extend diagonally across the beaks. Ligament very short, 
thin and of a lighter red than the epidermis, which is a dark-chest- 
nut color, with minute strie and with coarse growth-lines. Um- 
bonial slope obtusely angular. Posterior slope rounded on the 
margin and with a raised ridge line from beak to posterior end, 
forming there, generally, a slight biangulation. Dorsal area wide, 
cordate. Cardinal teeth large, erect, with furrowed sides, the ante- 
rior branch in the left valve being very thin and broad and with a 
long striate edge. Lateral teeth short, slightly curved, the outer 
one being the thinner. Anterior cicatrices distinct with a secondary 
cicatrix above, and running under the cardinal tooth. Posterior 
cicatrices distinct. Dorsal cicatrices few and under the posterior 
cardinal. Cavity of the sheil very deep, and quite uniformly hol- 
lowed out. Cavity of the beaks broad, roomy, the apex within 
being nearly a right angle. Nacre usually white, with a tint of 
blue, and iridescent at posterior end ; sometimes the nacre is entirely 
pink. 
Animal always white. Size, width 2 inches, length 1°75, diameter 
1:38. 
Habitat Bayou Pierre, an arm of the Red River in De Soto 
Parish, Louisiana. 
My Cabinet, U. S. National Museum, Academy of Natural 
Sciences, Phila. and cabinet of L. S. Frierson. 
