940 THE PALEONTOLOGY OF MINNESOTA. 



[.Raphistoma. 



RAPHISTOMA, Hall. 



Raphistoma, HALL, 1847, Pal. New York, vol. i, p. 28. 

 RapJiistoma (part.) and Pleurotomaria (part.), of many authors. 



For the generic characters and a full discussion of the relations of this genus 

 the reader is referred to the preceding remarks on the family. (See pages 931, 934.) 



Of the nineteen species referred to this genus in the last edition of Miller's 

 catalogue, perhaps only R. stamineum and R. planistria, with its variety parvum, 

 really belong here. The majority of the remainder must be classed as doubtful, 

 since we do not know whether they have a slit-band or not. Some may belong to 

 Raphistomina, as does R. lapicida Salter, or to Liospira, as do R. a'mericanam Billings sp. 

 (= Unticulare Emmons) and R. subtilistriatum Hall sp. R. niagarense Whitfield 

 certainly is not a Raphistoma. It seems to be congeneric with Meek and Worthen's 

 Platyostoma trigonostoma. Two good species of the genus were described by Billings 

 as Pleurotomaria calyx and PI. crevieri, both from the Canadian Chazy. To these we 

 add the two following new species. 



RAPHISTOMA PERACUTUM, n. sp. 



PLATE LXVIII, FIGS. 1-6. 



Shell small, not known to exceed 12 mm. in diameter, consisting of three or 

 three and one-half whorls, flat above, rounded below; periphery very sharp and thin; 

 umbilicus equalling a little more than a fourth of the greatest diameter, its edge 

 narrowly rounded; hight of shell very slightly exceeding a fourth of the width. 

 Surface marked on the flat upper side with very fine subequal striae sweeping on 

 the whole rather strongly backward from the suture. About a third of the width 

 of a whorl from the suture the striae are interrupted by a delicate revolving line. 

 Between the latter and the extreme edge of the peripheral carina the striae make a 

 distinct sigmoid curve. Below the periphery the surface is marked with similar 

 striae, which, in descending, curve first forward and then almost directly toward the 

 center of the umbilicus. 



In this species the hight is relatively less, and the peripheral edge consequently 

 thinner, than in any previously described species of this genus. It is the only 

 j^istropod known to us as occurring in the Lower Silurian rocks of Minnesota 

 having a perfectly flat spire. 



Formation and locality. Black Uiver group, Ctenodonta bed, Goodhue county, Minnesota. Rather 

 rare. 



Collections. E. O. Ulrich: W. H. Scofleld. 



