POST-GLACIAL MOLLUSCA 663 
Pisidium costatum Sterki 
Next to Pisidium tenuissimum calcareum this species is the 
most abundant mollusk in the collection. Of this species Dr. 
Sterki says, ‘‘For years I thought it might be a form of Pisidium 
medianum, but rather different; this find appears rather to vindi- 
cate it.”” It has not been observed among recent Pisidia and was 
previously known only from marl deposits in Michigan and Maine. 
With the previous species this southward distribution indicates 
a climatic change. 
Pisidium vesiculare Sterki 
A few rather small but characteristic specimens occur in the 
collection. This species is not known from Illinois as a living 
mollusk. It has been observed in the marls of Michigan. 
Pisidium variabile Prime 
One valve of this abundant species was found in the collection. 
It is a common species in the northern part of Illinois, both fossil 
and living, and it is strange that only a trace of its presence has 
been left in the bed at Urbana. 
Pisidium adamsi affine Sterki 
A single valve of a juvenile individual occurred with other 
Pisidia. It is known only from Winnebago County (collected by 
A. A. Hinkley). 
VALVATIDAE 
Valvata tricarinata (Say) 
Not common. The individuals are sharply tricarinate, though 
the majority are immature. This species is a common mollusk 
in the ponds, lakes, and streams of the northern part of the state. 
Valvata sincera (Say) | 
This valvata is quite common in the deposit. These are the 
first authentic specimens of sincera from an Illinois locality, 
specimens hitherto identified as sencera proving, upon examination, 
to be lewisii (see Baker, 1806, p. 91). The Urbana specimens of 
sincera were referred to Dr. Bryant Walker, who declared them to 
be quite typical of the species. This record indicates a wider 
southward distribution in early post-glacial time. 
