314 CONTIilBUTlONS TO CANADIAN PAL.'RONTOLOli Y. 



Ai/ni'sid. has been proposed by De Koniuck foi- sinisti'al species of this 

 genus, Ijut Lindstrom has shown that some species are both sinistral and 

 dextral. 



Rapiiistoma Tyrrellii. (N. Sp.) 

 Plate 41, figs. 5, oa, 6 and 6 a,b. 



Shell nearly flat on the apical side, oonvexly conical and imperforate 

 below, broader than high, spire very slightly elevated, periphery sharply 

 angulated and minutely crenulated. Volutions four or five, those of 

 the spire flattened aliove, their sides completely covered by the over- 

 lapping of those which succeed them, except at the anterior end of the 

 last \-olution but one, where a very small portion of the upper part of the 

 side is exposed ; outer volution shallowly concave above, its peripheral 

 angulation slightly produced on the apical side, in such a way as to form 

 a minute and not very prominent crenulated spiral ridge. Slit band placed 

 on the periphei-al angulation of the outer volution and of about equal 

 Ijreadth on each side of it : on the apical side it is concave, but not 

 separated from the rest of the surface Ijy any distinct bordering lines oi' 

 line, on the umbilical side it is flat but bounded externally by a very 

 minute spiral impressed line : crescents not distinctly defined, but producing 

 the minute crenulations on the periphery. 



Surface nearly smO(jth, marked only Ijy extremely faint lines of growth, 

 which curve convexly l)ackward to the slit band both above and behnv. 



Onion Point, Lake Manitoba, J. B. Tyrrell and J. F. Whiteaves, 18SS : 

 abundant, associated with Eiiumpludiin M/iii.iJiihp'ii.si.s, which is now 

 known to belong to the genus Oin;ph(ih>i',irrii.s, and both forms of I'liraryclan 

 elliiil'ira. Lake Winnipegosis, at Point Brabant, and on the south- 

 westei-n shore of Dawson Bay, at two small points, one about half a mile, 

 the other about three miles north of the mouth of Bell Biver ; also, on 

 iSwan Lake, near the mouth of Swan River, and on the I ted Deer River, 

 at the Lower Salt Sjiring, D. B. Dfiwling, 18SH : a few specimens from 

 each of these localities. Collected also by Messrs. Tyri-ell and Dowling, 

 in 1W89, at many other localities in or around Lake Winnipegosis, as on a 

 small island at the extreme south end of the lake, at Weston Point and 

 a small island oft' that point, at South Manitou Island, on the south- 

 west side of Camenjn Bay, and at eight exposui-eson the islands or shores 

 fif fJawson Bay. 



Most of the specimens are mere casts of the interioi' of the shell, but 

 three examples collected at Dawson Bay in IMS!) have the whole of the 

 test preserved, and it is from these that the foregoing descri])tion was 

 made. These internal casts, as shown by figs, (i, Ca and 6b on Plate xli, 

 are usually Ijroader, flatter and often nuich larger than the few te.stiferous 

 hiieeimens y<'t collected, such as the original of tigs, o and 5a on the same 



