942 THE PALEONTOLOGY OF MINNESOTA. 



[Raphlstomina lapiclda. 



concave, the amount and extent of the sinuate portion varying with the species. 

 Finally, the upper side of the shell is apparently never quite flat in Raphistomina, 

 while this is generally the case in Raphistoma. 



The shells of Raphistomina remind one in a general way very greatly of those of 

 our new pleurotomarian genera Liospim and Eotomaria. Still, when the surface 

 markings are preserved, no one is likely to have much trouble in separating them. 

 A glance should be sufficient for those accustomed to handling fossil gastropods. 

 The principal difference between these genera is that while Raphistomina has no 

 slit-band, both of the other genera have, Liospira having it directly at the peripheral 

 edge and Eotomaria immediately above it. 



Besides the four species figured in this work, three of which are new, we have 

 only one other species, Pleurotomaria laurentina Billings, which we can place here 

 with certainty. However, we can scarcely doubt that some of the lenticular shells 

 which, because they are insufficiently known, we must provisionally leave as 

 doubtful forms of Pleurotonuiria and Raphistoma, will eventually prove to belong to 

 this genus. 



RAPHISTOMINA LAPICIDA Suiter. 



PLATE LXVIII, FIGS. 18-20. 



Raphistoma lapicida SALTER, 1859, Can. Org. Bern., Decade 1, p. 12. 



Shell discoidal, nearly equally convex above and beneath the acute peripheral 

 carina, usually about 25 mm. in diameter and but little more than half as high. 

 Umbilicus deep, rather narrow, not abrupt, its width equalling about a fifth of the 

 diameter of the shell. Volutions about four, gently convex on both the upper and 

 lower sides except near the peripheral carina, where, especially upon the upper side, 

 the surface is rather distinctly concave. Excepting the last whorl, which sometimes 

 descends a little beneath the edge of the preceding, the spire slopes with very little 

 interruption at the sutures. The lines of growth are as usual somewhat stronger 

 below than above. As a rule they are irregular and unequal. On the upper side 

 they are turned backward from the suture but not very strongly, nor is the sigmoid 

 curve which they make a conspicuous feature. Still, the retral sweep is sufficiently 

 overcome in the concave region near the edge to cause them to intersect the latter 

 at nearly a right angle. On the lower side their course from the edge to the umbil- 

 ical cavity is nearly a straight line, a slight anterior curve just before they descend 

 into the umbilicus causing a faint sinus in the outer half. Aperture transverse, 

 acutely ovate, the outer extremity or angle turned slightly forward in a view from 

 below. Inner lip curved, slightly reflexed. 



