r,ASTi;<r<. 



tu ) 



Besides the species list-.l on page 852 there are three more American. or rather 

 i ana. lian. species that may belong to ' M These were descril.."l l.y Milling 



jis Helltri>hH ni.icn; II. p.t linn no. ami R <!//". tin- fir-t from the Calciferous forma 

 tion. the second from the Quebec group, the third from the Black River group. Of 

 the three form-, the first is the most likely to belong here, but in the absence of 

 any positive knowledge respecting their essential generic features, it is best to leave 

 them provisionally where they are placed by Mr. Billings. 



Respecting the systematic position of Oxydisctu, we are somewhat in doubt. 

 The absence of any sort of revolving lines is against the inclusion of the genus with 

 the liucaniidir. On the other hand such a position is strongly indicated by the 

 absence of lal.ial callosities and more particularly by the large umbilicus and long 

 apertural slit. The latter occurs most certainly in the Lower Silurian species next 

 described, and as these species agree verly closely in all respects with the Upper 

 Silurian and Devonian types &l the genus, we have assumed that the slit is one of 

 the prime characteristic* of the genus. Now, if this view proves to be correct we 

 cannot l>e far wrong in placing Ori/disctm in the immediate vicinity of CotirmMln. 

 'I'll-- only "litlerence of any consequence between these genera are that, while the 

 -nrface of the volutions of the latter are crossed by imbricating, wavy lamella*, 

 wliirh are not turned backward on the dorsum to form an angular sinus where they 

 meet at the keel, the whorls of the former are crossed simply by fine lines of growth, 

 turning backward very strongly in nearing the dorsal keel, their junction here 

 indicating a narrow, V-shaped excision in the outer lip. 



-THAI-ITUS, . *p. (Utrich.) 



I'l.VM I Alt. FIOB. B-: PLATE I.XXXII. FIO8.-. 



Shell lenticular; dorsum acutely carinated; greatest diameter from 15 mm. to 

 :> mm.; greatest thickness or width nearly one-half the diameter. Volutions three 

 ami one-half to four ami one-half, thickest near the umbilicus, from which the 

 surface ascends first with a gently convex, then with a concave slope to the sharp 

 arli volution embracing between one-third and one-half of the 

 preceding one; umbilicus exposing all the whorls; its width somewhat less than 

 one-third of the diameter of the shell; edge of umbilicus abrupt, subangulai : 

 aperture obcordate. indented below by the sharp dorsum of the preceding whorl; 

 margin of aperture thin, in a side view with a strong backward sweep; slit long, 

 very narrow. Surface marked by fine ami rather indistinct lines of growth. 



This species agrees perhaps as well as any with the Devonian types of the 

 genus. Of Silurian forms only 0. crwtatus Sallonl ami <>. .llsmfnx Hillings need be 



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