Geology of the St. Croix Dalles. — Berkey. 283 
the aperture under a new generic name. The forms collected rep- 
resenting this genus are so extremely variable among themselves and 
even on different parts of the same individual that it has been found 
inexpedient at this time to subdivide them very closely into species. 
Accordingly the greater number of specimens are grouped together 
under the specific name reciirviis already in use as defined by Whit- 
field. The particular individual or varietal form however v/hich W'.iit- 
fie!'' described and figured is not considered the best type of the 
genws. It seems to be an extreme or abnormal individual. There- 
fore one which is represented by several perfect casts v.as chosen in 
its place, (see plate XIX, figs, i and 2). I am of the opinion that M . 
retrosra Whitfield is essentially of the T)yblidiuin type and should 
net be transferred to the new genus. The apex of this specimen 
is defective and my experience with some of the peculiarities of these 
types leads me to believe that the recurved character of the apex is 
overdrawn in the reconstruction by Whitfield. It is believed that 
Metoptoina alta Whitfield, M. venilia Billings. M. oryt/iyia Billings, 
and perhaps others should be transferred to this genus. 
The shells of all specimens are quite thin. On the only specimen 
preserving a part of the shell it measures from .25 mm. to .45 mm. in 
thickness. On many others the original thickness is readily estimated 
by the separation of the walls of the cast and the results indicated 
in this way are very little greater than those given above. Variation 
in size is as great as in any other particular. The largest fragment 
indicates an aperture of more than 50 mm. through the longer axis. 
A portion of the cavity once filled by one of these forms has been 
estimated to require a shell over 100 mm. in length. 
Several of these specimens have a well defined slightly depressed 
area extending completely or almost completely round the cast usual- 
ly about one-fcnirth to one-thi'-d the distance from the base to apex. 
The persistence in occurrence and position of this band strongly sup- 
Iiorts the view that it represents the position of muscle scars of this 
genus. On several casts there is a circle of slightly raised areas lying 
in this position on the cone. On only a few casts are these well pre- 
served but in all such cases the marks are the same in form and po- 
sition and number. It is therefore added as a character of the genus. 
— that the muscle attachments form a circle of six pairs of scars con- 
siderably above the aperture and parallel to it. 
Description. Shell conical, high; apex smooth and more or less 
curved or recurved toward or even beyond the broader margin of the 
oval aperture: aperture entire and more or less acuminate anteriorly; 
surface smooth or striated; muscle scars in six pairs forming a circle 
parallel to the aperture and about one-third the distance from base to 
apex. 
