256 THE AMERICAN MIDLAND NATURALIST 
when charged, ventral edges distended, water canals present, 
no specialized structure of mantle edge antero-ventrad to branchial ° 
opening; glochidium large, broadly spadiform, spined, hinge 
line straight. 
SHELL CHARACTERS :—Shell thin, subalated, smooth on disk ; 
beaks flat, sculptured with four or five rather double-looped ridges ; 
epidermis smooth, polished, rayed in green in the region of the 
post-umbonal ridge; hinge teeth absent; scars faint, confluent; 
nacre bluish. 
In this state this genus is represented by the two species, 
Las. ohiensis (Raf.) and suborbiculata (Say)—the latter not having 
been completely described hitherto. The author has had conve- 
nient access to large beds of suborbiculata and has been fortunate 
in securing specimens gravid with embryos in all stages and with 
mature glochidia. Neither has the latter been figured nor des- 
cribed before. Because of the fact that the marsupium of subor- 
biculata is more like that of Arcidens and that of ohiensts closer to 
Anodonta we would group the latter as more modern; then, too, 
the hermaphroditism and longer breeding season of ohiensis 
would also indicate an advance in being able to perpetuate the 
race. 
Lastena suborbiculata (Say). 
(“‘Suborb,”’ ‘‘ Heel-splitter.’’) 
PI. IV, Fig: 19a; Pl. 1X, Fig. 19; Pl. XXIII, Figs. 73 A—D: 
1831—Anodonta suborbiculata Say, New Harm. Diss. (Newspaper 
form); Am. Conch. I, No: Il, 1831 (Later date), p. 2x1: 
1867—Anodon suborbiculatus Sowerby, Conch. Icon., XVII, Pl. V, fig. 11, 
[it 
ANIMAL CHARACTERS. 
NUTRITIVE STRUCTURES:—Branchial opening comparatively 
small, upward curved with many fine orange colored papillae; 
anal also directed upward, smooth with Y-shaped yellow markings; 
supra-anal long, far removed from anal by mantle connections; 
inner gills wider but very little longer than outer, inner laminae 
of inner gills not connected to visceral mass; palpi rather long, 
united antero-dorsad about one-third of their length; pericardinal 
region very large, watery, pinkish-brown in color; foot, long, 
thin, deep orange in color, adductors also orange, yellowish re- 
tractors and protractors visible through the watery, transparent 
