GASTROPODA. KM 1 



>rlnn It will be observed that the umbilicus does not descend as abruptly as In that species, the 

 convex slope belli* much inure Inclined giving a relatively greater width to thi> umbilicus at lu narrowly 

 rounded margin. 



Var MACBA. n. rar. 



PLATE I.XX V. FIDS. IS and 1*. 



ast upon which this variety Is bated, waa found at a lower horizon than that which hold* the. 



typical form. So far u we can see it differ* only In being more depressed, the transverse and vertical 



diameters of the last whorl near the aperture being to each other respectively an four Is to three. In the 



i; f. .mi the same dimensions are as four Is to live. Part of this difference may bo due to distortion 



Indeed we think It Is, since at the inner end of the outer whorl the two dimensions are almost e<iual. 



Formation and locality. -Ot the typical form we have four specimens, from the Maclurea U-d of the 

 Trenton group, of which one was obtained from each of the following localities In Minnesota: Lime City, 

 Slewartvllle, Pleasant Grove and WykofT. One of these belongs to the Survey museum, the others to E. 

 O. Ulrlch. Var. mami was collected by the latter In the Puslsplra bed at Hader. 



Mttitum Register, No. " 



Genus HACLURINA, n. gen. 



This genus is proposed in accordance with our remarks on page 1038, for the 

 reception of shells heretofore classed as Maclurea, but differing from the typical 

 form of the genus in wanting the projections for the attachment of muscles on 

 the inner side of the operculnm. Maclurea manitobensis Whiteaves, the operculum 

 of which is figured and described by Whiteaves in the Canadian Record of Science 

 for April, 1893, is regarded as the type of the new genus. In this species the 

 nucleus is at the junction of the lower and inner margin of the operculum, and 

 we believe the same is true of M. cuneata and M. subrotunda of Whitfield, which 

 with Whiteaves' species, are all that at the present time it seems safe to refer 

 to Murliirinn. Billings says of the operculum of bis M. oceana that it has no 

 muscular process, and he also figures the opercula of two otherwise unknown 

 -["'ies which likewise are without such projections. But these opercnla differ 

 uo wiilely from that of M. manitobensis that it seems highly improbable that they 

 can belong to shells of the same genus. 



M\I.ii:iNV MANITOBENSIS WhitCOVtS. 



IM.\n I.\\\IH..- i :,r.! I'IMI I \XXII. FIO. 46. 



Macliirra manitobenn* WHITEAVES, 1800, Trans. Roy. Soc. Can., vol. vll, Sect. 4, p. 76; also 1803, 

 Canadian Record <, p. 324. 



Original dtfcriptio*. "Shell large, attaining to a maximum diameter of eight Inches and a half, and 



consisting of about flve somewhat slender volutions which increase rather slowly lo size : <>ut<-r volution 



nearly always distinctly angulated at the periphery. Left (in. almost flat, but faintly depressed 



In the c.-nt.T In oome specimens and as faintly raised In others: volution, as viewed on the flat side, very 



-66 



