884 THE PALEONTOLOGY OF MINNESOTA. 



[Bucanfa. 



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 has a 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 iipon the rate of increase in size of the volutions, being 

 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 Bucaniidce 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. 



