20 
SILURIAN 
minute sixth, which only gradually increase in size—the outer 
whorl being but one-fifth wider than the preceding. The 
whorls are plane, or only slightly concave near the margin, 
along which a fine thread or ridge runs, a little way within 
the edge. The lines of growth are greatly curved backward 
to the margin, and are very fine, thread-like, and regular. 
But little can be seen of the convex underside, which, how¬ 
ever, appears to resemble that of many other species. 
It is like a Itaphistoma in its shape, and in the direction of 
its lines of growth, which run to the extreme edge, crossing 
obliquely the sub-marginal ridge. 
But if the direction of the lines were only changed so much 
as to be at right angles to this ridge, we could not hesitate 
to call it a Pleurotomaria , so closely allied are the extreme 
forms of that genus to Raphisioma. Yet this seems to me no 
reason for regarding the latter as otherwise than a true gen¬ 
eric type. All the Raphistomce have this lenticular shape, and 
all want the true band ; while in Pleurotomaria very few 
species put on the lenticular form, and all have the true broad 
sinus defined on either edge. 
The genera without a band, Raphistoma and Helicotoma 
(see “ Decades of the Canadian Survey ” No. 1, pi. 2,) form 
a'small group, parallel to that of the genera with a distinct 
band— Pleurotomaria, Murchisonia, Hormotoma. 
I think the genus Raphistoma a natural one, and therefore 
do not coincide with the opinion of my friend Mr. Billings, 
who considers all these forms as sub-genera of Pleurotomaria. 
Locality. —Chorhoti Pass, 17,000 feet. (No. 1744.) 
RAPHISTOMA. .-Bp. 
Plate 2, fig. 13. 
The proportions of the whorls are nearly the same as in the 
last species, but they are decidedly less flattened, and less acute 
in section, as seen in the enlarged figure 13 a. 
Locality. —Chorhoti Pass, 17,000 feet. (No. 1744.) 
