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 Pleurohema 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 obliquum- 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 obliquum 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 



