842 THE PALEONTOLOGY OF MINNESOTA. 



IStenotheca exsorta. 



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. magnified,, 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. 

 Colkction. Geological and Natural History Survey of Minnesota. 

 Museum Register, No. 5535. 



Genus STENOTHECA, Salter. 



Stenotheca, (SALTEK) HICKS. 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 Patellidce. 

 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 



