84 



This species has a sharper eruargination at the lateral angle of the 

 aperture than is shown for H. tenuistriatus. Billings speaks of a similar 

 notch in his species H. princeps, but Mr. Walcott does not depict this 

 feature in the examples he ascribes to Billings species. The Cape Breton 

 species differs from //. excellens, Bill., in the sharper lateral angles of the 

 tube, &c. 



Besides this species there are fragments of the tubes of Hyolithes at 

 three other horizons of the lower Etcheminian ; they are, however, in poor 

 preservation, and we have not been able to recognize the opercula of any 

 form of this or the preceding genus in the Etcheminian rocks of Cape 

 Breton. 



In the Upper Etcheminian of this island we have not found the remains 

 of any Hyolithidw, though it is apparently in this part of the Etcheminian 

 terrane that they occur at Smith's Sound in Newfoundland. The genus 

 Orthotheca is exhibited there in great variety. On that sound the Etche- 

 minian beds consist of fine argillaceous sediments of a red color, with 

 some thin limestone beds, while in Cape Breton the Hyolithes-bearing 

 beds are mostly coarse and sandy and abound in effusive volcanic materials. 

 The difference in the genesis of the sediments in the two regions may ac- 

 count for the perfect condition of the fossils in the one, and their frag- 

 mentary state in the other. 



1. Development of the genera Acrothyra, Acrotreta and AcrotJiele. 



Development ^ e va/ l ue ^ small species of fossils in determining geological horizons 



of the Genera is well shown in Tullberg's monograph on the Agnosti, of which genus 



Acrotreta and certain types are peculiar to special horizons of the Cambrian and of the 



Acrothele. Ordovician. A small fragment of rock only has been found sufficient, 



when containing certain Agnosti, to determine the age of a group of strata. 



I hope it may hereafter be possible to use the three genera above men- 



tioned in a similar way for determining the age of parts of the Etche- 



minian and the higher Cambrian, where these genera occur. It is as a 



contribution to this object that the writer presents here descriptions of 



such species and varieties as have been recognized in the Canadian 



Cambrian rocks. 



It will be seen that so far as our knowledge goes, the first two genera 

 are among the oldest that have been recognized in the Cambrian rocks of 

 Eastern Canada, since they are found along with the volcanics that lie at 

 the base of the Palaeozoic terranes, as well as higher up in the Cam- 

 brian ; and they were distinct from each other, even at that early time. 

 The following table shows the distribution of these early forms of 

 Brachiopods in the Basal Cambrian rocks and their relative abundance at 

 Dugald brook at the several horizons at which they occur ; 



