MOLLUSC A FROM INDIANA AND OHIO 447 



Pisidium tenuissimum Sterki. The most abundant species of 

 Sphaeriidae in the deposit and quite typical of the species. This 

 Pisidium seems to be a common Pleistocene fossil, occurring in 

 widely separated deposits in Maine, Michigan, Ohio, and lUinois. 

 In the deposits at Urbana, IlHnois, believed to be pre- Wisconsin, 

 a variety of this species — calcareum Sterki — is the most abundant 

 moUusk in the material examined (see Baker, 1918, p. 663). It is 

 quite significant that tenuissimum has not yet been found in 

 deposits older than post-early Wisconsin; none are recorded 

 from Sangamon or Peorian deposits. It is absent from the 

 Indiana deposits discussed in this paper and beheved to be of 

 Sangamon age. 



Pisidium medianum Sterki. A score of Pisidia, with small, thin 

 shells and very prominent beaks, are referred to this species by 

 Dr. Sterki. 



Pisidium novehoracense Prime. Pisidium pauperculum Sterki. 

 Two valves each of the foregoing species were identified from the 

 material by Dr. Sterki. They are both typical of the species. 



Valvatidae 



Valvata tricarinata (Say). This is one of the most abundant 

 species in this marl deposit. The majority of the specimens have 

 three strong raised keels and are in every way typical of the species. 

 In a few forms, however, the central carina is faintly developed, 

 showing a variation toward the next variety. 



Valvata tricarinata perconfusa Walker. About 10 per cent of 

 the carinate Valvatas belong to this variety. There is some 

 variation in the smooth space between the carinae on the shoulder 

 of the whorl and on the base of the shell, there being in some 

 specimens a faint ridge indicating the position of the central carina 

 in typical tricarinata. 



Valvata tricarinata unicarinata DeKay. A single specimen of 

 DeKay's variety was found among several hundred tricarinata. 

 This variety appears to be rare among both fossil and recent 

 members of the species. 



Valvata sincera Say. Three specimens of this characteristic 

 species were picked out of a quart or more of marl specimens 



