THE NATURAL HISTORY SURVEY. 91 
marsupium, having distinct ovisacs much like Lampsilis; 
inner gill partly free from abdominal sac; mantle often fringed 
in front of branchial opening. (Simpson. ) 
GROUP OF PLAGIOLA DONACIFORMIS. 
24. Plagiola donaciformis Lea, pl. xiii., fig. 4. 
Unio donaciformis Lea, Trans. Amer. Phil. Soc., Vol. III., p. 266, pl. 
iv., fig. 3, 1827. Male. 
Unio zigzag Lea, Trans. Amer. Phil. Soc., Vol. III., p. 409, pl. xii., fig. 
19, 1829. Female. 
Shell: Elliptically elongated, thick and solid, inflated; dor- 
sal margin almost straight; ventral margin rounded; rounded 
before, acutely biangular in the male and somewhat truncated 
in the female; surface smooth and shining, lines of growth 
indicated by obtuse ridges; umbones not much elevated but 
much inflated, yellowish or greenish in color, marked by very 
many fine elevated ridges; anterior umbonal slope rounded; 
posterior slope long, slightly excavated, shorter in the female 
than in the male, and forming a rounded ridge or shoulder; 
ligament short, rather wide,dark horn color; epidermis light green 
or yellowish green, with numerous rays composed of V-shaped 
dashes; in the female the dashes are frequently larger and form 
somewhat continuous rays; cardinal teeth double in the left and 
single in the right valve, small, elevated, narrow, more or less 
triangular, coarsely serrated; the single tooth in the right valve 
is acutely triangular; lateral teeth long, directed ventrally, 
lamelliform, almost straight, rather thick, striated; the right — 
lateral has a small depression near the posterior end; bridge 
between the teeth arched, narrow, thick; anterior adductor 
muscle scar longer than wide, deeply excavated, striated; pos- 
terior adductor muscle scar ovate, scarcely visible, confluent; 
protractor pedis muscle scar wider than long, deeply impressed, 
striated; dorsal muscle scar situated in the cavity of the beaks, 
numerous, deeply excavated; pallial line slightly impressed; 
cavity of the beaks rather deep in most specimens; nacre sil- 
very white, iridescent, especially on the posterior part. 
Length, 48.00; height, 32.50; breadth, 28.00 mill. © (8366). 
Animal; Marsupium occupying posterior half of outer 
gill of female in numerous distinct ovisacs; branchiz small, 
inner much the larger, nearly semigircular; free from abdominal 
sac only at posterior end; mantle with many brown papille on 
postbasal edge; branchial and anal openings small, each with 
