239 



Description. — Shell small ; Avidth from 1^ to 2 inches ; spire flat, of 

 four or five slender whorls, which, as seen in the casts of the interior, are 

 uniformly convex on the upper side, the suture being deeply impressed. 

 Umbilicus somewhat more than three-fifths the whole width; the edge, as 

 shown in a vertical polished section, acute ; the inner sides of the whorls, 

 in. the umbilicus, convex ; all the whorls seen, in staircase form, to the 

 apex. Surface unknown. 



Width 18 to 24 lines ; height 6 to 8 lines ; width of last whorl 5 to 6 

 lines. 



Differs from M. Ummonsi in having more slender whorls. It also 

 resembles M. ponderosa, but this latter is vastly larger. 



In perfect shells the whorls may be flat above, although in tlje casts 

 they are rounded. 



Locality and Formation. — F, Keppel Island, Newfoundland ; Quebec 

 group. 



Collector. — J. Richardson. 



Maclurea ponderosa. (N. sp.) 



Description. — Shell large ; from four to six inches across ; of four or 

 five whorls. Spire flat, the central whorls often a little elevated above 

 the outer ones ; the margin obtusely rounded. Base truncated hemis- 

 pherical ; umbilicus about half the whole width, with an acute edge ; 

 sides of the whorls in the umbilicus moderately convex. Surface with 

 strong squamose striae parallel to the edge of the aperture. Shell remark- 

 ably thick, usually exhibiting a tendency to exfoliation. 



No very perfect specimens of this species have been collected, although 

 it occurs in vast numbers in several localities. The edge of the umbilicus 

 in ordinary specimens is acutely rounded, but in good vertical polished 

 Sections it is very acute. Such sections also show that, in some, the edge 

 of the whorls is exposed, in the umbihcus, in staircase form, to the apex, 

 but in others it is quite certain that it (the edge) is concealed by the 

 preceding whorl. 



In most specimens the whorls on the upper side are uniformly 

 depressed convex, with a moderately deep suture, but sometunes they are 

 nearly flat. 



A specimen of four whorls is about 5 inches across. 



This species, in size and proportions, much resembles M. Magna, but 

 difiers from it in having the whorls a little more slender in the horizontal 

 direction, and deeper in the vertical, in having the edge of the umbilicus 

 more acute and always a thicker shell. The vertical section is alinost 

 exactly like that of if. affinis, as represented above in fig. 224, which, 



