THE NATURAL HISTORY SURVEY. 75 
brown. The outer gills were distended with young April 12, 
1897, and were of a deep chocolate color. Glocidia, as well as 
eggs in the morula stage, were very numerous, the former very 
active. Normally all four gills are used as marsupia. 
Distribution: Ullinois and Michigan to Ohio and western 
New York. 
Geological distribution: Pleistocene. 
Habitat: In small rivers and creeks, on a muddy bottom. 
Remarks: This species may be distinguished from 4. 
' ferussacianus, with which it is very closely related and may be 
but a variety, by the greenish gold color of the umbones and its 
general cylindroid shape. The males are not so cylindrical as 
the females, and have the posterior end very pointed while in the 
female it is rounded. The species also varies in color, some 
being plain yellowish brown, while others are greenish yellow 
rayed with grass green. Itis not as common as /erussacianus, 
and is found in the Desplaines River and its tributaries, the 
north branch of the Chicago River and Lake Michigan. Some 
forms are of a uniform yellowish horn color, without rays of any 
kind. 
* K ** 
Ovules filling all four gills of female, ovisacs not separated 
by a sulcus. (Simpson.) 
Genus QUADRULA Rafinesque, 1820. 
Shell: Solid, triangular to rhomboidal with a well devel- 
oped posterior ridge and generally dark or only feebly rayed 
epidermis, and in age often showing a tendency to become 
arcuate on the base; beaks high, usually curved forward, their 
sculpture consisting of a fewrather heavy ridges, either nearly 
parallel with the growth lines or showing a tendency to fall into 
an anterior and posterior loop; hinge plate heavy, with the teeth 
arranged much as in Plagio/a; cavity of the beaks deep and 
compressed; muscle scars well defined, anterior scar deep. 
There is often in this genus a decided lunule under and just in 
front of the beaks which is most conspicuous in the more solid, 
inflated forms. 
Animal: Anal opening with only minute papille or none; 
all four gills of the female being transformed throughout into 
marsupia when gravid; ovisacs not separated bya sulcus; inner 
gill generally free from the abdominal sac. (Simpson. ) 
