NEW OSTRAGODERM— 0RVIG H 



have a truncated, or slightly rounded, posterior margin (pi. 1, figs. 

 3-6) . Others, which probably were situated further backwards in the 

 crests, are equally as long as broad, and their posterior margin is more 

 or less obtusely V-shaped. In all the ridge-scales one may, just as 

 in those of Astraspis desiderata, distmguish the following faces: (a) 

 a convex external face, consisting of an anterior portion which slopes 

 forwards and downwards and which is overlapped by the ridge-scale 

 in front (oda, pi. 1, figs. 3, 4), and a posterior, exposed portion orna- 

 mented with tubercles throughout its extent (pi. 1, figs. 3, 4); (b) a 

 concave basal face having the appearance of a broad, longitudinal 

 groove (bf, pi. 1, figs. 5, 6); (c) a concave posterior face overlapping 

 the anterior portion of the external face of the ridge-scale following 

 next behind (ogp, pi. 1, figs. 5, 6) ; and, finally, (d) a paired lateral face, 

 a little concave and sometimes fau-ly low, overlapping the adjoining 

 polygonal plates on each side (oQi, pi. 1, figs. 3, 5). As fully evidenced 

 by their structure, the ridge-scales were firmly attached to each other. 

 In the material at my disposal, however, there is no indication what- 

 soever that they tended to fuse with their fellows in the individual 

 crests in the same manner as in Astraspis desiderata (0rvig, in MS., b). 



The scales presumably belonging to the trunk behind the carapace 

 are far less common in the material than the polygonal plates of the 

 carapace and the ridge-scales. They are more or less rhomboid in 

 shape, frequently broader than long, and deeply imbricating. Theu- 

 external face is made up of a large anterior overlapped portion and a 

 fairly narrow posterior exposed portion ornamented with tubercles. 

 Scales of this particular kind are, as far as I can find, not met with in 

 the Astraspis material from the Harding Sandstone or from other 

 formations. 



The ornamentation of the plates and scales is subject to considerable 

 variation. Some of the elements that are clearly preserved in early 

 stages of growth show an immature type of ornam.entation, made up 

 of a shagreen of fairly small and low tubercles (fig. 2, a, cf. ta, ph 1, 

 figs. 1, 2, 4), which in superficial view are rather similar to those on 

 the exoskeleton of various Psammosteida and certain Osteostraci as 

 well Csee Obrutchev, 1956, pi. 1, fig. 4). These tubercles are highly 

 reminiscent of those in Astraspis (fig. 4; Walcott, 1892, pi. 3, figs. 13, 

 14; Bryant, 1936, p. 418, pi. 2, fig. 2), and are, in fact, only distinguish- 

 able from the latter by the circumstance that, in the individual plates 

 and scales, they are remiarkably uniform in size and, as a rule, placed 

 close beside each other with about equal interspaces. They consist 

 of a bulbous crown, exhibiting deep grooves radiating in a basal direc- 

 tion from the apex, and a fairly low neck-portion which is somewhat 

 constricted below the crown but otherwise not sharply defined from 

 the latter. 



