166 G. LINDSTROM, ON THE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



side of the operculum on the limestone or shale, which forms the nucleus of the shell. 

 That it is only a cast and not the operculum itself, may be found by comparing the 

 inside, drawn in fig. 9 of Etheridge's plate, with fig. 14. Quite similar impressions 

 have been found in several specimens in Gotland. The Oriostomata figured by the 

 Etheridge in figs. 10 — 11, much resemble 0. sculptum, but the opercula seem to deviate 

 in shape as well as in sculpture from that, which with absolute certainty is known as 

 belonging to that species. The operculum, fig. 13, is possiblj' identical with one from 

 Hogran. Mr Fred. Smithe has also given a notice with figures »0n the opercula of 

 the Silurian Gastropoda" in the Proceedings of the Cotteswold Naturalists Field Club 

 for 1877 — 78 p. 6"2. The specimens figured are evidently badly preserved and belong 

 to some unknown species, the shell of which resembles Or. sculptum, but the sections 

 given of the operculum show (figs. 3 — 4) that it cannot belong to that species. 



Baily^) has given a figure of the operculum of »Euomphalus funatus Sowerby» 

 but it seems to be different from that .described above, and rather more loosely coi- 

 led, resembling that in fig. 44 pi. XVII of the present memoir. 



Lately Whiteaves') has described two opercula, of which, at least one, pi. Ill f. 



II & pi. VII f. 7, comes near to that of Or. coronatum. The second operculum, pi. 



III f. 10, again, is nearly related to those of Cyclomena described below. 



7. Oriostoma acutum n. 



PI. XVII fig. 37—40. 



Shell globose, turbinate, with spire of six ventricose whorls, angular through the 

 projecting, narrow, revolving keels. On the apical surface there are three keels, sepa- 

 rated by large interspaces, on the umbilical side they are much crowded, amounting to 

 ten before the first umbilical ridge is attained. Between this and the interior umbili- 

 cal ridge some five or six smaller ridges lie in a gently excavated, large groove. The 

 innermost ridge closely environs the deep and narrow umbilicus. All these revolving 

 keels are almost smooth, with sharp or rounded edge, only finely notched by minute, 

 transverse strias. These are in the interstices directed backwards and so fine that only 

 a few, regularly distantiated ones are perceptible without a lens. Between them the sur- 

 face looks nearly smooth, but is in reality finely striated. The aperture is circular 

 and both lips are thin. H. 37 mill., br. 45 mill. 



But for the find of the operculum of this shell in situ, it might easily enough 

 have been mistaken for a variety of 0. globosum, from which it, however, differs 

 through the minute, transverse striae, the regular angularity of its apical side and the 

 regularity of the umbilical funnel. The operculum is acurainately conical, nearly of 

 the same height as breadth. The inner side is almost flat, the bordering edge is nar- 

 row and low, the central whorl is nearly on a level with the other surface and through 

 corrosion any sculpture, if formerly extant, has been destroyed. The coils on the out- 

 side are exceedingly narrow and close. They are formed like the sharp, thin, knife like 



') Figures of Charact. Brit. Fossils pt. Ill pi. 21 fig. 9 b. 



^) Geol. Survey of Canada. Palfeoz. Fossils, vol. Ill pt I, p. 3,3. 



