62 THE CHICAGO ACADEMY OF SCIENCES. 
all species by its squarish, heavy shell and peculiar, double 
looped umbonal sculpture, this last character being peculiar to 
the species. The female may be known by its more swollen 
shell, which is very much rounded on the ventral border. The 
very young shell (museum number 10086) shows the character- 
istic marking of the umbones better than the adult form. (Plate 
ix., fig. 4.) Pearls are frequently found in these species. 
A single female specimen, from the Little Calumet River, 
was examined April 6,1897. This specimen was very broad 
and extremely alate. In this specimen the entire outer gills 
were used as brood pouches, and the glocidia were very numer- 
ous and active, closing their delicate shells frequently with 
a sudden jerk. They measured .30 mill. in length, and the 
byssus, adductor muscle and hooks on the valves were very 
much developed and conspicuous. There isa great difference 
between the male and female shells, so much so that they might 
easily be mistaken for different species. The Little Calumet 
River is the metropolis of this form, where it is largely and 
finely developed. 
GROUP OF ALASMODONTA MARGINATA. 
7. Alasmodonta marginata Say, pl. iv., fig. 4; pl. vii., fig. 7; pl. xxii., fig. 3. 
Anadonta marginata Say, Nich., Encycl., ed. 1, Vol. II, pl. iii, fig. 5. 
1816. 
Shell: Smooth or slightly wrinkled, inflated, more or less 
quadrangular, rounded before and triangular behind, the apex 
truncated and placed near the ventral margin; dorsal margin 
curved, ventral margin straight or curved imward; umbonal 
slopes broadly rounded, the anterior angle short and the posterior 
angle long, wide and almost flat; surface marked by lines of 
growth which are raised in various places, especially on the 
posterior angle; umbones large, a trifle elevated, inflated, dark 
brown in color and marked by three very coarse, undulating, 
elevated ridges, and a single feeble one, each ridge forming two 
slightly curved loops, all directed anteriorly; ligament wide, 
not very long, very dark horn color; epidermis brownish or 
greenish, with wide darker rays, extending from the umbones to 
the ventral border, and dotted with black spots and dashes in 
various places; the rays are not soconspicuous in old specimens 
as in young ones; cardinal teeth not heavy, thin, elevated, 
striated, one in the right and two in the left valve; lateral teeth 
represented by a thickening of the hinge plate; anterior adduc- 
