448 Fossils of the Chazy Limestone, 



with the posterior margin, while the latter is only gently convex 

 until within one- third of the base, then rounded to the lower mar- 

 gin. The wing is rather prominent and u scarcely at all compressed. 



Diagonal of the largest specimen fifteen lines. 



A specimen of this species without the wing is rather acutely 

 oval, and has an aspect very different from the perfect form. 



I have placed it in the genus Vanuxemia provisionally, but 

 it may be necessary hereafter to remove it to some other genus. 



Locality and Formation — Island of Montreal and near L'Orig- 

 tial. Chazy Limestone. Not common. 



Collectors. — Sir W. E. Logan and R. Bell. 



Gasteropoda, 2 1 species. 



There are in the Lower and Middle Silurian rocks of Canada 

 upwards of thirty species of Gasteropoda that must be distributed 

 among the genera Pleurotomaria Scalites and Raphistoma, pro- 

 vided these groups be retained as distinct from each other. But 

 after giving the subject a great deal of consideration, I cannot sec 

 that the last two are possessed of any structural peculiarity of 

 sufficient importance to warrant a separation from the first. It 

 has always been supposed that Scalites and Raphistoma were 

 destitute of a spiral band, yet the species which I have called 

 P. docens has a band as strongly marked as it is in any known 

 species of Pleurotomaria, while it has also a nearly flat spire, a 

 largely developed conical base, and no umbilicus. The three 

 latter characters, combined with the peculiar aspect of the shell, 

 shew that we cannot separate it generically from Scalites, and 

 yet the spiral band connects it with Pleurotomaria. This fossil 

 had not been discovered at the time of the publication of Decade 

 I., otherwise a view of the affinities of Scalites different from that 

 put forth in the work would have been maint lined. 



In general the fossils of this group are not well preserved in 

 our rocks ; but in the large collections of the Geological Survey 

 there are specimens of many species retaining the surface mark- 

 ings, and it is quite clear that all have either a spiral band, or a 

 sharp bend in the lines of growth which is equivalent thereto. 

 In connection with the band there is also in all the species a 

 notch in the outer lip, and there cannot be the least doubt but 

 that the band was formed by the progressive filling up of the 

 notch during the growth of the shell, precisely as in the genus 

 Pleurotomaria. 



