884 THE PALEONTOLOGY OF MINNESOTA. 
[Bucania. 
choice would, we believe, have fallen upon the expansa,—and it would have made a 
good type of a good genus. But, since Roemer has described a genus that will 
include the expansa, and both Waagen and Koken take B. sulcatina, the first species 
following Hall’s description of the genus, as the type,—a course that is fully justi- 
fied,—it would not be good policy, and only add to confusion where there is 
too much already, if we were to revert to what evidently was Hall’s original 
intention. Nor can we blame either Roemer or Waagen for overlooking or ignoring 
his intention, since in the absence of a more complete knowledge of B. expansa than 
was furnished by Hall, they would not have been justified in departing from the 
rule which, when the type is not designated, gives that rank to the first species 
following the generic description. 
As we are all agreed to adopt B. sulcatina as the type and to restrict the genus 
to species having essentially the same characters as that species, the first thing to 
do is to determine exactly which are the essential peculiarities and which are 
not. This may not have been possible for either Waagen or Koken, and we are 
probably the first having sufficient material, both in the way of species and individ- 
uals, to do it in an approximately satisfactory manner. 
The amended description of Bucania given on page 850 rests on no less than 
twenty Lower Silurian species. These show that the umbilicus is nearly always 
large, certainly never small. The outer lip hasa broad V-shaped sinus and a central 
slit, the slit-band is narrow and slightly elevated, flat, or channel-like. The surface 
markings run in two directions, transversely and spirally, and both sets cross the 
volutions obliquely from the umbilicus to the slit-band, the degree of obliquity of 
the spiral lines depending upon the rate of increase in size of the volutions, béing 
greatest in those in which the expansion is the most rapid. The transverse 
(growth) lines are oblique because they curve in directions parallel with the margin 
of the aperture. But the feature of the surface sculpture that deserves the most 
attention, and this applies to the Bucaniide as a whole, is that the intersections of 
the two sets of lines are nearly always rectangular. 
According to the remaining characters, the species fall into two groups, the 
first or typical section, containing the type of the genus and seven other species, 
being characterized by a thin shell, very broad umbilicus, slowly and gradually 
enlarging depressed volutions, relatively wide yet not expanded aperture, thin lips, 
and long slit. In the second or B. lindsleyi section the shell is thicker, the umbili- 
cus smaller, the volutions enlarge more rapidly and are higher, the inner lip is 
thicker and slightly reflected on each side, and the slit shorter. These differences 
will be better understood after a comparison of figures 1-12 with the remaining 
figures on plate LXVI. 
