CANADIAN FOSSILS. 13 



last species, four of them in the diameter of half an inch ; they are 

 not quite flat above, but gently convex between the sutures. Below 

 they are strongly angular at about the middle of the volution, the 

 outer face being plane, and the umbilicus broad and conical, with 

 slightly curved sides ; its width is fully half the entire diameter of 

 the shell. 



The mouth is rhomboidal, as high as wide, the outer angle about 

 80°, its lower and inner margin defined by the angle and sloping 

 wall of the umbilicus ; the margin not reflexed. 



This has a more convex base than the R. (Pleurotomaria) subtili- 

 striata (Hall*), which nevertheless appears to be of the same genus. 

 P. lenticularis, Emmons,t also resembles ours, but it is a larger shell, 

 and less angular below. 



R. aperta is however so like R. ( Euomphalus ) qualteriata of Schlot- 

 heim from the Lower Silurian limestones of St. Petersburg, that but 

 for the greater size and less elevated spire, it might be taken for the 

 young of it. R. qualteriata however seems constantly to be more 

 depressed above, and the outer angle therefore of the mouth not so 

 equal-sided as ours ; nor is the umbilicus quite so broad or so 

 decidedly angulated. 



There are several other species of Raphistoma in the Canadian 

 limestone, one an inch and a half in diameter, slightly concave at the 

 sutures, and with large angular umbilicus. 



Locality. — Allumette Island. 



Helicotoma. 



The characters are given above, and only four species are yet 

 known — one the H. (Euomphalus) wiiangulata of Hall, from the 

 Calciferous sandrock of New York, and the three species here given. 

 if. planulata should stand for the type of the sub-genus, which is less 

 regularly depressed than Raphistoma, and without the reversed umbi- 

 licus of Ophileta, though with a much larger one than the other 

 sub-genera. 



* Palaeontology of New York, vol. L, plate 37, figure 5. 



t Geology Second District, New York, page 393 (not of Sowerby). 



