8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. io8 



There has been some difference of opinion in recent years concerning 

 the correlation of the Bighorn formation with Ordovician strata else- 

 where in the United States (see, e. g., Twenhofel et al., 1954). Ac- 

 cording to what has been said above, however, there can be little 

 doubt with regard to the dating of the red fossiliferous shale which, in 

 the Rock Creek section, follows on top of the massive dolomite mem- 

 bers of this formation. Despite the fact that their exact position in 

 the section cannot, at the moment, be made out with certainty, it is 

 clear that the plates and scales of Pycnaspis splendens, new genus, 

 new species, are no older than these red shales and that, therefore, 

 they are of an Upper Ordovician age. It is true that there are also 

 outcrops of red vertebrate-bearing sandstone beds underneath the 

 Bighorn Dolomite in the Rock Creek section similar to those further 

 south in the same area (see E. Kirk, 1930, p. 462; Amsden and Miller, 

 1942, fig. 1) but, according to Dr. Berdan (in litt.), there is no evidence 

 at this locality of tectonic movements of such an order as to bring 

 these sandstone beds in contact with the upper part of the Bighorn 

 formation. 



Remarks: In the other material at my disposal, I have been able 

 to identify some plates of Pycnaspis sp. indet., to some extent rem- 

 iniscent of those of Pycnaspis splendens, new genus, new species, both 

 in the sandstone and shale of the Winnipeg formation of the Williston 

 Basin in Montana (pi. 2, figs. 1-3) and in a phosphatic conglomerate 

 from the lower portion of the "Icebox" shale member of the White- 

 wood formation of the Black Hills region in South Dakota (White- 

 wood Creek section, near Deadwood). On the other hand, Pycnaspis 

 plates and scales are missing altogether in all the numerous samples of 

 the Harding Sandstone of Colorado which I have had the oppor- 

 tunity of examining in American and European museums, a circum- 

 stance which may be of some importance from the point of view of 

 stratigraphy. 



The specific name refers to the shining crown of the tubercles of the 

 exoskeleton. 



Description: In the material of Pycnaspis splendens, new genus, 

 new species, one may distinguish thick exoskeletal plates and scales of 

 mainly three different categories: (a) polygonal plates from the 

 carapace; (b) ridge-scales, for the most part, at least, from the cara- 

 pace; and (c) other scales probably belonging to the trunk behind the 

 carapace. The polygonal plates and ridge-scales are in several respects 

 rather reminiscent of those in the carapace of Astraspis desiderata 

 (0rvig, in MS., b), and everything goes to prove that they originally 

 occupied much the same position in relation to each other as in the 

 latter form. Thus, the ridge-scales very likely are detached com- 

 ponents of a system of longitudinal crests extending, like in Astras- 



