97 



These are described in. some detail in a paper published in the Canadian 

 Record of Science for December, 1891, and entitled " Notes on the occur- 

 rence of Paucispiral Opercula of Gasteropoda in the Guelph formation of 

 Ontario." As there stated, they are " rather thin, nearly flat, but slightly 

 concave externally and as slightly convex internally, broadly subovate, 

 about one-fifth longer than broad, obtusely pointed at the end correspon- 

 ding to the posterior angle of the mouth of the shell whose aperture they 

 closed, paucispiral and composed of from two and a half to three rapidly 

 expanding volutions, the nucleus being eccentric."* " Only the outer or 

 concave surface of each of those opercula is exposed to view, the inner 

 side being buried in the matrix." Figure 6 on Plate 14 represents the ex- 

 terior of the largest and most perfect specimen known to the writer, of 

 natural size. Its maximum length is twenty millimetres, and its greatest 

 breadth sixteen. It is so highly dolomitized that it is difficult to estimate 

 its exact thickness, but at the distance of a millimetre from the edge, at 

 the somewhat truncated termination of the outer volution, its thickness 

 is between one-half and three-quarters of a millimetre, though it seems 

 to increase rapidly inward. 



These opercula would seem to have belonged to shells that are referable 

 eith&r to the Naticidfe or to the Littorinidse. At present there is no 

 satisfactory evidence that the Naticida? dated as far back in time as the 

 Palaeozoic era, but the Littorinidse are said to be represented in the Silurian 

 period by numerous species of Holopea. It has previously been shown 

 that four nominal species of Holopea have been described from the Guelph 

 formation in Canada, but the generic relations of H. Guelphensis are still 

 very obscure, and it is most probable that 11. occidentalis is a Polytropis 



B. Supposed multispiral opercula. 



Plate 15, figs. 4, 5, 5 a and 6. 



Several specimens of an organism, which may be the operculum of some 

 gasteropod, were collected at Elora and Durham by Messrs. Boyle and 

 Townsend between the years 1878 and 1892. The most perfect of these 

 organisms, such as the specimen represented by figure 4 on Plate 15, are 

 elongate conical, nearly three times as long as broad and truncated at the 

 larger end. Their internal structure, as shown in natural or artificial 

 longitudinal sections, like that represented three times the natural size 

 and somewhat diagrammatically by figure 6 on the same Plate, is very 

 singular. It consists of an extremely elongated, slender, multispiral cal- 

 careous and solid central axis {a), which is everywhere surrounded by a 

 second and correspondingly slender, solid and tightly enveloping calca- 



*Ilather than " subcentral, " as stated in the paper referred to. 



