406 Canadian Record of Science. 



ing woodcut represents the exterior of the best specimen 

 known to the writer, of natural size. Its maximum length 

 is twenty millimetres and its greatest breadth sixteen. 



Figure 1. Paucispiral operculum of a gasteropod, genus and species unknown, 

 from the Guelph Formation of Ontario. 



It is at present quite impossible to determine to which of 

 the known gasteropoda from the Guelph formation in 

 Ontario these opercula should be referred, if, indeed, they 

 are referable to any. Judging by the shapes of the aper- 

 tures of the shells into which they may have fitted, the 

 most likely species, perhaps, are the Holopea gracia or H. 

 harmonia of Billings, or a small and undescribed naticoid 

 shell from Durham, which, so far as can be ascertained 

 from a few casts of the interior, seems to be closely related 

 to the Holopea nux of Lindstrom, from the Upper Silurian 

 of Gothland. The resemblance of the operculum here 

 figured to that of Litorina and Natica is very striking, and 

 in this connection it is to be noted that Lindstrom places 

 H'Aopea in the L/torinidae. In the recent species of Litorina 

 the opeiculum is invariably chitinous and extremely thin, 

 while in Nutica proper it is calcareous and not nearly so 

 thin. The one here figured is so highly dolomitized that it 

 is difficult to estimate its exact thickness, but it gives the 

 writer the impression of being thicker than that of a recent 

 Litorina. At the distance of a millimetre from the edge, 

 its thickness, at the somewhat truncated termination of 

 the outer volution, is between one-half and three-quarters 

 of a millimetre, but it seems to increase rather rapidly in 

 thickness inward. 



The only other opercula known to the writer as occurring 

 in the Palaeozoic rocks of Canada are the depressed multi- 



