of inflation is suliject to slight variability. Ihe scales show a 

 tendency to alternate on adjaoei-.t rows anri except near the ventral 

 margin of arliilt shells the rows are not seoaratecl by definite 

 grooves, '-^he majority of the specimens have nineteen ribs* !I!he 

 Internal lirations usually eirtend over more than half of the dis- 

 tance to the tunbo, althoxip-h on thick shells the distance is less. 

 Yoiing shells are less orbicular than adults, being higher than 

 •"ide, the sculpture on the interspaces is reduced to radial, rais- 

 ed scaly lines, the ribs are merely roughened and their distal 

 ends squarer, 



■^'his species is recop'nized by the scabrous character of the 

 sculpture and the almost ^oristant number (nineteen) of d is tally 

 subrounded ribs. It is closely related to the Pliocene and Recent 

 P. exasperates Sowerby (1), v.-ith which Gu.ppy first confused it. 



(1). Thesaur. CJonch., vol. 1, p. 54, pi. 18, figs. 18.^, 184, 186, 

 (=?. f us c o pn.r pur e us Oonrad, 184:9), 



bu.t the recent form is larger and in^iivi duals of the same size 

 are usually less orbic:lar, more inflated, with steeper dorsal 

 margins, the scales arranged in more definite radial series and 

 more prominent in the interspaces. Brovm and Pilsbry have des- 

 cribed two varieties of this species; as oxyn-onus canalis (2) from 



( r: ;> Proc. Acad. I'at. 3ci., Philadelphia, vol. 64, p. 511, ol. 

 24, fie-. 5, 1012. 



