THE NATURAL HISTORY SURVEY. 93 
silvery white, sometimes tinged with light lavender, iridescent, 
especially on the posterior part. 
Length, 36.00; height, 31.00; breadth, 17.00 mill. ( 1768). 
iE BRO: (5 a eee OD ely fF) 1800-108 E-LZ6B), 
of 22.005)? 39.00; a 24.00 (13440). 
Animal; Anal opening small, with dark papille; branchial 
opening with a few black papille; mantle thin, with a wide, 
thickened border which is double and s//gh¢/y toothed below; 
inner gill the larger in front, united throughout to the abdomi- 
nal sac; outer gill larger behind; marsupia occupying the pos- 
terior part of the outer gill in about fifty ovisacs with a longi- 
tudinal fold far above its base. (Simpson. ) 
Distribution: Mississippi and Ohio Valleys and Lake 
Michigan. 
Geological distribution: Pleistocene. 
Hlabitat: Similar to donaciformis. 
Remarks: This is a species which is frequently confounded 
with P. donaciformis. Ithasa much more trigonal shell and the 
ziguzag character of the rays differs from donaciformis. In 
the latter species the zigzags are large and coarse and the V-s 
long, and scattered over the surface in connected festoons; in 
elegans the zigzags are small, the V-s short and placed in a reg- 
ular disconnected series of rays. When once known the two 
species need never be confused. In some rare specimens the 
rays are not composed of zigzag rays, but are straight and 
thread like. It is not a common species, and is restricted to the 
southern region. 
Genus LAMPSILIS Rafinesque, 1820. 
Shell: Oval to elongate, inflated, moderately solid, with- 
outa clearly defined posterior ridge, and never becoming arcuate 
with age, with a hard, bright, usually rayed epidermis; beak 
sculpture consisting for the most part of delicate parallel ridges 
which show a tendency to fall into an anterior and posterior 
loop. Inthe female the postbasal part of the shell is swollen 
opposite the part of the branchize which contains the embryos. 
Hinge containing two cardinals (pseudo-cardinals) and two 
laterals (pseudo-laterals) in the left valve, and one cardinal with 
sometimes a small second one above, and one lateral in the 
right valve. 
Animal; Branchial opening papillose, anal opening gen- 
erally having well developed papille; inner gills usually united 
