38 ILLINOIS BIOLOGICAL MONOGRAPHS {136 
In the Sangamon River coccineum is a most abundant and variable 
mussel, having a smooth, polished shell on which the rays are many and 
distinct. It also attains a large size, though not as large as specimens 
from the Big Vermilion drainage. The shape of the shell is more oblique 
and eliptical than is the species as it occurs in the Big Vermilion and the 
shell is a trifle more inflated on the average. So marked is the difference 
that it is comparatively easy to say from the shape of the shell and the 
surface markings what drainage a particular individual may have come 
from. The twelve sets of coccineum in this collection indicate in a marked 
degree the fact that species may differ conspicuously in both sculpture 
and form in different river systems. The nacre of the Sangamon River 
coccineum is more often white than in the Big Vermilion shells. Pearly 
secretions or pathologic malformations have not been observed in the 
specimens from the Sangamon River, indicating, without doubt, a more 
favorable environment than is provided by the waters of the Salt Fork. 
Young specimens (25-30 mm. long) are more abundant in the Sangamon 
River than in the Big Vermilion River, and these individuals are beautifully 
marked with dark green rays on a yellowish or light brown background. 
Occasional shells are pinkish. The beak markings on the umbones are 
especially well preserved in these young specimens. 
A large right valve from Mahomet is very peculiar. In outline it is 
ovate, a trifle oblique. The posteroir portion of the valve is much elongated 
the hinge line is long and straight, and the posterior margin is sharply, 
obliquely truncated. The umbonal region is near the anterior margin of 
the valve. The lateral tooth is longer and straighter than in normal 
coccineum. The shell recalls Plewrobema clava but is much larger and 
differently shaped. The valve measures as follows: length 92, height 
70 mm. 
Ortmann (1918:549) considers coccineum a variety or race of obliquum 
(Conrad), together with solidus(=catillus Conrad), which is also rated as 
a variety of obliquum. To this disposition the writer cannot agree, the 
forms here listed as varieties being quite as much entitled to specific rank 
as are many other forms recognized as distinct species which have marked 
variation and a similar facies. The whole group of obliguum-solidum- 
coccineum are closely related, but I have seen no good reason after 
examining a large series in the Hinkley and other collections in the 
Museum collections, for lumping these species as varieties of obliquum. 
As far as Illinois specimens of obliguum and coccineum are concerned, the 
two species seem sufficiently distinct for recognition. 
10. Rotundaria tuberculata Rafinesque. Purple Warty-Back. 
This species was found at but two places in Salt Fork, at Homer Park 
and South of Muncie, and in the Big Vermilion below Middle Fork. At 
