842 THE PALEONTOLOGY OF MINNESOTA. 
[Stenotheca exserta. 
shell and produce corresponding lines on the cast. Diameter about 55 mm.; hight 
about 21 mm. 
This species is not nearly as high as S. superba and S. magnifica, while it differs 
from S. beloitensis in having coarser radii, and from S. obtusa in the more central 
position of the apex. 
Formation and locality—Trenton group, Clitambonites bed, St. Paul, Minnesota. 
Collection.—Geological and Natural History Survey of Minnesota. 
Museum Register, No. 5535. 
Genus STENOTHECA, Salter. 
Stenotheca, (SALTER) Hicxs, 1872, Quart. Jour. Geol. Soc., vol. xxviii, p. 180. 
For generic characters see page 822. 
The typical species of this genus are from the Cambrian, from which horizon 
some ten or twelve species have been described. From these it would appear that 
the generic type is distinguished from Scenella chiefly by the curved form and 
stronger concentric marking. The genus seems, however, even among the Cambrian 
forms, to be subject to considerable variation in the matters of form and surface 
marking, and in such ways that we consider ourselves justified in placing the two 
species about to be described within its limits. Neither of the latter is greatly 
different from certain varieties of the Cambrian S. rugosa as figured by Walcott. 
Stenotheca, as is the case with Scenella also, is often placed with the Pteropoda. 
We fail, however, to see anything in these shells to justify such a view, at any rate 
nothing that is not overcome by evidence favoring an alliance with the Patellide. 
We must admit that Stenotheca is not a good member of this family, but it most 
probably represents an offshoot from Scenella, which is a better example, toward 
certain bellerophontoids (e. g. Cyrtolites). 
STENOTHECA EXSERTA Sardeson. 
PLATE LXXXII, FIGS. 11—15. 
Tryblidium exsertum SARDESON, 1892, Bull. Minn, Acad. Nat. Sci., vol. iii, p. 337. 
Shell high, laterally compressed, curved, forming one-third or more of a volution; 
aperture subovate, the length and width about as three is to two, more narrowly 
rounded in front (beneath the apex) than behind. Surface marked with fine 
radiating lines, increasing in strength with the growth of the shell, with, so far as 
known, not less than two in the space of 1 mm. Obscure transverse markings are 
also present and on the basal half of a large cast several broad folds. On the 
specimen referred to we fail to see any signs of the radiating lines, but on the other 
