838 THE PALEONTOLOGY OF MINNESOTA. 



IScenella 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 Palccacmcea. 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 Tryblidium 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 Palwacmwa 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. alia 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. M. 



Metoptoma superba BILLINGS, 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 striae, 1 mm. or less in width, and concentric lines and 

 obscure wrinkles. In casts both sets of striae 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 

 32 mm. In the largest specimen seen the aperture is nearly circular and between 

 85 and 90 mm. in diameter, the hight about 45 mm. 



