238 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 52. 



Genus ASTRASPIS Walcott. 



ASTRASPIS DESIDERATA Walcott. 



Plate 12, figs. 5, G. 



Aslraspis desiderata Walcott, Bull. Geol. Soc. Ainer., vol. 3, 1892, p. 166, pi. 3, 

 figs. 6-14; pi. 4, figs. 1-4. 



The original specimens upon which this species was founded are 

 preserved in the United States National Museum collection, and are 

 catalogued under the number 2351. They consist of fragmentary 

 plates, ornamented mth a coarse tuberculation, and not sufficiently 

 complete to permit of even orcUnal determination. A suggestion 

 was made, however, by the original author that the form was allied 

 to Devonian Antiarchs Uke Asterolepis ornata. 



Some months after the presentation of Doctor Walcott's paper 

 before the Geological Society in 1891, and before it had been printed 

 in the bulletin, a unique and extremely important specimen of As- 

 trajns was discovered at the type locality, and a brief description of 

 it was given by Doctor Walcott in a footnote dated March, 1892, 

 added to page 167 of his paper. This specimen is now preserved in 

 the Museum collection (Cat. No. 8121) and is illustrated for the first 

 time in the accompanying plate 12, figure 6. Preserved in the form 

 of an impression of the outer surface, a plaster cast taken from the 

 natural mold is shown in figure 5 of the same plate. 



The later discovered specimen shows a structural characteristic 

 which the earlier known fragments failed to disclose, namely, that 

 the large element, or shield, is of compound nature, being made up 

 of a large number of small polygonal tesserae in precisely the same 

 mamier as in Cephalaspids and Psammosteids. Moreover, the style 

 of ornamentation is similar to that observed in the f amihes just named, 

 each of the small polygonal tesserae rising into a conspicuous central 

 prominence which is surrounded by numerous minute stellate tuber- 

 cles. The compound nature of the shield was recognized by Walcott, 

 and the tuberculated ornament was compared by him with the some- 

 what similar features displayed in Thyesfes verrucosus Eichwald, in 

 which the larger tubercles are disposed in several longitudinal rows. 

 Influenced by this consideration, and also by a resemblance in general 

 outline, Walcott reached the conclusion that Astj^aspis was related on 

 the one hand to ''cephalaspidian fishes of the Silurian of Russia," 

 and on the other to "Asterolepidae of the lower Devonian." ' 



The large compound plate of Astraspis was homologized by Walcott 

 with the head-shield of Cephalaspids, although it fails to exhibit any 

 trace of orbits and other prominences characteristic of that group or 

 of other members of the Aspidocephalous order of Ostracoderms. 

 Because these features are lacking in the impression of the plate be- 



BuU. Geol. Soc. Amer., vol. 3, 1892, p. 167. 



