122 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



species in the higher beds of the sandstone and in the succeeding 

 niagnesian limestone. The lower beds are especially characterized 

 by the presence of Conocephalites ; but I hesitate to admit, 

 among these, the occurrence of the Genus Dikelocephalus. Not- 

 withstanding the late determination by Dr. Shumard of the D. 

 latifrons from the lower beds, I cannot avoid tlie conclusion 

 that this too, with its "truncated conical'' glabella, is a Cono- 

 cephalites.* In the intermediate beds, however, we have the 

 Dikelocephalus ^niniscacnsis of Owen, which he cites from the third 

 Trilobite bed of F 1, two hundred or two hundred and twenty feet 

 below the top of that formation. In all the specimens obtained, 

 I have failed to find one of this species with a facial suture 

 corresponding to typical Dikelocephalus, notwithstanding that 

 some of the specimens appear to preserve entire the central por- 

 tion of the ceplialic shield. The direction of the suture is more 

 nearly as in Arionellus, and the places of the eyes more anterior 

 than in Dikelocephalus iniimesotensis or D. pepinejisis ; while the 

 cheek is short and broad, and little extended at the posterior 

 angle. It appears to me, therefore, that this species should be 

 admitted into the latter genus only after satisfactory evidence of 

 its character shall have been obtained. The typical species, D. 

 minnesotensis of Owen, is cited as occurring in member d of F 1, 

 *' ninety or one hundred feet below the base of the Lower Mag- 

 " nesian limestone, near the margin of Lake St. Croix above Still- 

 ^' water ; towards the base of Lagrange mountain, and at the 

 ** great slide beloAv Lake Pepin, Avhich is the fifth trilobite bed 

 *' of the series of F 1." The D. pepinensis is found in the same 

 association as the preceding. 



The interval between the lower beds at Mountain island, and 

 those in Avhich these two species of Dikelocephalus occur, is be- 

 tween four and five hundred feet. This interval may be represented 

 by a greater or less thickness in other parts of the group. It may 

 have been accumulated in a comparatively short time, or it may 

 represent a long period ; for although some parts of the series 

 appear to have been rapidly accumulated, there are lines of de- 

 markation indicating a change in the nature of the sediment,, and 

 other evidences of beach and shore lines with fucoidal remains, 

 together with mud-cracks and other marks of desiccation, so 

 that we cannot be sure that the accumulation was equal and 

 constant. It may therefore represent an interval, which, in some 

 other part of the country, will be found occupied by a much 



* See remarks under the generic description of Conocephalites, p. 13137. 



