838 THE PALEONTOLOGY OF MINNESOTA. 
[Scenella superba. 
through the shell so as to be visible on the outer half of casts of the interior, 
but usually the casts are smooth, or exhibit only a few obscure concentric wrinkles. 
The muscular scars of the typical species,—it is a Cambrian fossil,—are unknown, 
but in several Lower Silurian forms, which with our present light on the subject 
must be regarded as congeneric, they are very much as in Palwacmea. Though 
they have not been observed- very clearly, it is almost certain that they are 
imperfectly coherent and form a sort of wreath about the apical portion of the cast. 
The number of the scars is not established. Whitfield says that there are eight pairs 
in his T'ryblidium conicum, a species which we believe to belong to this genus and very 
near S. (Metoptoma) montrealensis Billings. In our S. beloitensis the scars seem to 
have been divided into four sets of which the two in the posterior half appear to 
consist each of four almost completely coherent imprints. 
When the surface markings are not preserved species of Scenella are dis- 
tinguished from those of Palwacmea by their higher conical form. In Stenotheca 
the shell is still higher and the apex curves far forward, projecting in most cases 
considerably beyond the anterior margin of the aperture. While these characters 
will suffice ordinarily in discriminating between the genera, there are still several 
forms of which it is difficult to decide whether they should be placed under Scenella 
or Stenotheca or arranged by themselves. Metoptoma venilla Billings, Quebec group, 
and M. alta Whitfield, Calciferous group, are examples of these doubtful species. 
Scenella probably ranged through the greater part of the Paleozoic rocks, the 
oldest occurring in the Lower Cambrian, while the most recent known representative 
appears to be the Devonian species which Walcott has described from Nevada 
as Metoptoma devonica. 
ScENELLA SUPERBA Billings. 
PLATE LXI, FIG. 35. 
Metoptoma superba BrLLiNes, 1865, Pal. Foss., vol. i, p. 172. 
? Conchopeltis minnesolensis WALCOTT, 1876, 28th Rep. N. Y. Mus. Nat. Hist., p. 93. 
Shell large, subconical, the hight usually equalling somewhat more than half 
the diameter; aperture broadly ovate or circular, slightly arched in front and 
behind; apex subcentral or a little anterior, obtuse, inclining gently forward. Sur- 
face marked by radiating striz, 1mm. or less in width, and concentric lines and 
obscure wrinkles. In casts both sets of striz are usually very obscure, while the 
radiating lines are seldom shown and then only for a short distance above the 
margin. Dimensions of an average example: length 64 mm.; width 58 mm.; hight 
32mm. In the largest specimen seen the aperture is nearly circular and between 
85 and 90 mm. in diameter, the hight about 45 mm. 
