Paucispiral Opercula of Gasteropoda. 405 



tion of Ontario, 1 are circular in outline, their inner surface 

 being flat, or nearly flat, and their outer surface convex. 

 They vary considerably in the amount of their external 

 convexity, some being nearly hemispherical and others 

 conical externally, and probably belong to more genera 

 than one. By analogy with similar specimens that have 

 been found in place, in shells of the genera Polytropis, 

 De Koninck (=Oriostoma, Munier Chalmas), and Cyclonema, 

 Hall, in the Upper Silurian rocks of Gothland, these multi- 

 spiral opercula from Durham are presumed to belong to 

 species of those genera, the Euomphalus macrolineatus of 

 Whitfield, and the Straparollus crenulatus of the present 

 writer, both of which occur at Durham, being now known 

 to be referable to Polytropis, and the genus Cyclonema to be 

 represented at Durham by the C. sulcatum of Hall, though 

 this latter shell also may be a true Polytropis. Both Poly- 

 tropis and Cyclonema are referred by Lindstrom to the fam- 

 ily Turbinidos, partly because their shells " have retained 

 the most evident traces of a nacreous layer," and partly on 

 account of their solid calcareous opercula. 



About five or six years ago, a few opercula of an entirely 

 different character to any of those already mentioned were 

 collected by Mr. Townsend in the G-uelph formation at 

 Durham. These, so far as the writer has been able to ascer- 

 tain, are so unlike any opercula that have hitherto been 

 described as occurring in palaeozoic rocks, that it is thought 

 desirable to place a short description of them upon record. 

 They are rather thin, nearly flat, but slightly concave exter- 

 nally and as slightly convex internally, broadly subovate, 

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

 end corresponding to the posterior angle of the mouth of 

 the shell whose aperture they closed, paucispiral and com- 

 posed of from two and a-half to three rapidly expanding 

 volutions, the nucleus being subcentral. Only the outer or 

 concave surface of each of these opercula is exposed to view, 

 the inner side being buried in the matrix. Theaccompany- 



!" Geological and Natural History Survey of Canada. Palaeozoic Fossils," 

 vol. Ill, pt. 1, Montreal, 1884, p. 33, pi. iii, figs. 10, lOa-b and 11, and pi. vii, fig. 7. 



