32 



NATURAL HISTORY BULLETIN 



Radula: The radiila shows 1 row of central teeth, and 9 rows 

 of lateral and 19 rows of marginal teeth on each side. The outer 

 one of the two side cusps of the laterals is quite uniformly larger. 

 (See fig. V.) 



Its nearest relative in the upper Mississippi valley is S. avara 

 from which it differs by its larger size, the more flattened body- 

 whorl, the distinct spiral grooves, the blunt and enlarged ends of 

 the jaw, and the nine rows of lateral teeth of the radula, each 

 lateral with the outer side cusp larger. It is nearer 8. ovalis in 

 size, but differs in being less inflated, with flattened and spirally 

 marked body-whorl, and with a simpler jaw of the 8. avara type. 

 It also equals 8. retusa in size but is more oblique, spirally 

 marked, and with a higher spire. 



The specific name is given in honor of the late Professor F. M. 

 Witter of Muscatine, who devoted many years to the study of 

 Iowa mollusks. 



Distribution and liahits: The species has been collected by the 

 writer for many years in the vicinity of Iowa City, Iowa, where 

 it is locally common. Some years ago specimens w^ere sent out 

 under the name of 8. avara var. vermcta. The species is gre- 

 garious on mudd}^ borders of the Iowa river. Where the mud 

 has been recently exposed, and is still quite soft, numerous in- 

 dividuals may be found creeping about, sometimes reaching the 

 water's edge. 



The types which are figured were selected from two sets : the 

 first six on the plate and in the table of dimensions are from a set 

 which was collected at the town of Coralville, near Iowa City, and 

 the remaining twelve specimens were collected in Iowa City, — in 

 both cases along the Iowa river. The jaw and radula were taken 

 from the Coralville specimens. 



The types are in the writer's collection, and cotypes have been 

 deposited in the zoological museum of the State University of 

 Iowa, the National ]\Iuseum, and the Philadelphia Academy of 

 Natural Sciences. 



