16 PROCEEDINGS OF THE NATIONAL MUSEUM vol. io8 



Other vertebrates 



The great majority (at least 75 percent) of the plates and scales in 

 the material from the Rock Creek locality at my disposal belongs to 

 Pycnaspis splendens, new genus, new species, but there are also several 

 (about 20 percent) referable to the Eriptychiida, probably to a new 

 genus of this group, and some few to Astraspis? sp. In addition, the 

 fauna may also include exoskeletal elements of other forms whose 

 systematic position is at present indeterminable. On the other hand, 

 the Pycnaspis-heSirmg beds do not seem to contain pieces of bone 

 tissue with cell-spaces or pieces of globular calcified cartilage, like 

 those met with in the Harding Sandstone of Colorado (0rvig, 1951, 

 p. 381, fig. 18a) and the Winnipeg formation of the Williston Basin 

 in Montana. To what extent those beds are devoid of other micro- 

 fossils, such as lingulid shells, conodonts, etc., cannot be decided with 

 any certainty on the basis of the limited material dealt with here. 



The plates belonging to Astraspis^. sp. (fig. 4) are of approximately 

 the same order of size as those of Pycnaspis splendens, new genus, new 

 species, with the mature type of ornamentation, but clearly distin- 

 guishable from the latter by the shape and microstructure of their 

 tubercles (fig. 2,&, pi. 1, figs. 1-4; cf. also pi. 2, figs. 1-3). These 

 tubercles, which vary considerably in size, all consist of a pointed 

 crown with deep, radiating grooves, and a conical neck-portion, which 

 is frequently comparatively high. In their microstructure, they are 

 similar to those in Astrapis desiderata (Bryant, 1936, pp. 418-420, 

 pi. 2, fig. 3; pis. 3, 4; pi. 5, figs. 1, 3; Prvig, 1951, fig. 22b; m MS., a; 

 Gross, 1954, pi. 3, fig. 2; pi. 5, figs. 3, 4) and those belongmg to early 

 stages of growth in Pycnaspis splendens, new genus, new species. 



Most of the material of Eriptychiida gen. and sp. indet. comprises 

 fragments of plates, frequently of large size, whose ornamentation 

 consists of fairly low and broad tubercles. These tubercules (fig. 5,a)— 

 which as a rule are considerably coarser than those of the Eriptychius 

 plates from the Harding Sandstone of Colorado (Walcott, 1892, pi. 4, 

 figs. 8, 9; Bryant, 1936, pi. 8, fig. 1) and the Winnipeg formation of 

 the Williston Basin in Montana (0rvig, in MS., a) — are fairly low, 

 elongated pearshaped or irregular in outline, and smooth on top; they 

 have no well-defined neckportion and they are frequently arranged 

 in more or less distinct rows (pi. 2, figs. 4, 5, 7). As in Eriptychius 

 (fig. 5,b,c; Bryant, 1936, pi. 11) and in the post-Ordovician Heterostraci 

 in general, these tubercles consist entirely of ordinary dentine. In 

 their basal part, there is no large pulp cavity but a system of vascular 

 canals from which the dentinal tubes issue in a superficial direction (for 

 further details concerning the microstructure, see Prvig, in MS., a). 

 In only one of the plates is the position in the cuirass of the living 



