NEW OSTRACODERM— 0RVIG 



13 



upwards and are blunt on top (fig. 2,h, pi. 1, fig. 2). On the other 

 hand, in the ridge-scales of the longitudinal crests and in the polygonal 

 plates which probably occupied a position in the interspaces between 

 these crests on the posterior portion of the carapace, the tubercles 

 are inclined backwards (pi. 1, figs. 3, 4) and are frequently tapering 



Figure 4. — Astraspisl sp. Large exoskeletal plate from the Pycnaj-^u-bearing beds of the 

 Rock Creek section, in oblique lateral view. (USNM 21338; X20.) 



to a point posteriorly. In Astraspis desiderata from the Harding 

 Sandstone there is little or no indication of a smilar variation in the 

 shape of the tubercles from one part of the carapace to another 

 (0rvig, in MS., b). 



The plates and scales of the exoskeleton are all remarkably stout, 

 with their average thickness being, as in Astraspis and Eriptychius, 

 probably greater in relation to the size of the animals as a whole than 

 in any of the geologically younger members of the Heterostraci. 

 They are built up of three layers — a superficial tubercle-layer; a 

 middle, vascular layer; and a basal layer. In their general propor- 

 tions these layers correspond closely to the similar layers in the exo- 

 skeleton of Astraspis (Bryant, 1936, pp. 418-421, pi. 2, fig. 3). 



The tubercles are made up of two, sharply defined, histological 

 constituents: (a) a thick upper layer of compact, semitransparent, 

 sometimes faintly bluish white substance {ezul, hzul, fig. 3) which, as 

 I shall have the opportunity of showing in another connection (0rvig, in 



