WHITEAVES] DEVONIAN FOSSILS OF MANITOBA, ETC. 341 



(S.) Pleurotomaeia Spencehi. (N. Sp.) 



Plates 41, figs, 4, 4a-b, 43, fig. 13, and 4G, fiff 1. 



Shell depressed subturbinate, more than twice a.s broad as high, ovately 

 orbicular as seen from above, spire low and short : base concavely depressed 

 in the centre, but apparently not umbiHcated in such a way as to expose 

 any portion of the inner whorls. Volutions four, expanding rather gra- 

 dually in a lateral direction, those of the spire depressed convex, the outer 

 one occupying three-fifths of the entire diameter, somewhat flattened, 

 both above and below, and depressed next to the suture above : periphery 

 narrowly rounded : suture distinct. Aperture obhque, transversely ellip- 

 tical, witli the peristome interrupted on the columellar side, the inter- 

 ruption being caused by the encroachment of the narrow peripheral 

 portion of the preceding volution ; lip with a notch or slit oljscurely 

 indicated on the apical side of the periphery, but otherwise thin and 

 simple, its inner margin shallowly concave. 



Surface marked with numerous, closely and rather regularly disposed 

 minute spiral ridges, which are crossed by still more minute lines of 

 growth, parallel to the outer lip. These incremental striiv are too small 

 to be seen without the aid of a lens, and ar'e not shewn on any of the 

 figures on Plates xli or xliii. They are, however, represented somewhat 

 diagiammatically in fig. 1 on Plate xlvi. Slit band not clearly observed, 

 though its presence is inferred, partly from the obscure notcli or slit on 

 the outer lip at the periphery and partly from the backward curvature of 

 the lines of growth, both above and below. 



Maximum breadth of the largest specimen collected, twenty-five milli- 

 metres : approximate height of the same, as measured in the median line 

 of the dorsal surface, about ten mm. 



Western shore of Dawson Bay, Lake Winnipegosis, " from slabs appar- 

 ently derived from the neighbouring cliflis," J. W. Spencer, 1^74: one 

 specimen, the original of the figures on Plate xli. A few smaller and 

 less perfect specimens, six in all, were collected by Messrs. Tyrrell and 

 Dowling in 1889, in place, at exposures on two small islands in the 

 southern part of Dawson Bay, and on the western shore of that bay, at 

 the mouth of the Red Deer River and on the first small point north of 

 the mouth of the Red Deer River. Small specimens, whose maximum 

 diameter is about eight or nine millimetres, which are probably the young 

 of this species, were collected by Mr. Tyrrell and the writer, in 1888, at 

 Pentamerus Point, Lake Manitoba, and by Messrs. Tyrrell and Dowling 

 in 1889, on three small islands in the southern portion of Dawson Bay. 

 One of these specimens (from Pentamerus Point), is represented, twice 

 the natural size, on Plate xliii, fig. 13. At this stage of growth the 



