172 Natural History Bulletin. 



at position of carina; epidermis ; suture distinct, deeply 



impressed only at commencement of last whorl and below, 

 above last whorl covered by carina; aperture narrowl}^ ovate, 

 broadly rounded before and narrowly rounded behind, very 

 oblique; peritreme continuous, almost rimate, slightly reflected 

 over the parietal wall; lip simple, sharp." (Call and Pilsbry.) 



In commenting on P. vifsshsippfensis^ Messrs. Call and 

 Pilsbry say: "It differs in important particulars from the P. 

 scalar if or mis^ Wolf, which is 'carinate its entire length,' has 

 a different aperture, and is markedly different in the character 

 of the sutures. Our species is ecarinate on ail whorls above 

 the body-whorl, the carina being depressed and filling entirely 

 the suture. The outlines of the apical whorls are wholly 

 unlike the figured type of Wolf's species. The average 

 measurements of the four specimens upon which the preced- 

 ing description is based are, for length, 1 4.66 mm.; for breadth 

 1. 61 mm." 



The writer has collected more than 1500 of these little 

 snails, and a careful comparison ol the specimens in this large 

 series shows that the characters which are cited above as dis- 

 tinguishing between P. niississippiensis and P. sccihin'fonnis 

 are not constant and have no specific value. 



Four specimens of this series are represented in Fig. 3. 



Of these a and b are more nearly t3'pical scalar if or mis ^ while 

 c and d are the forms included in niississippiensis. Tlie series 

 however is an unbroken one, the species varying from the 

 ecarinate forms which are scarcely angled at the periphery 

 of the body-whorl, and in which the suture is not impressed 

 (f/. Fig 3) to forms in which the carina is elevated and 

 extends quite to the apex, and in which the suture is deeply 

 impressed. (^, Fig. 3.) The form of the aperture also varies 

 with the prominence of the carina, and in some specimens the 



I Th2 dim Elisions of thass four specimens, as recorded separately, sliow 

 tbiat the avera^^e len^jth is 3.46 mm. instead of 4.66 mm. Probably a typo- 

 graphical error. 



