6 PROCEEDINGS OF THE NATIONAL MUSEUM vol. los 



Wetting Agent 326) immediately before the first thin latex coat ia 

 applied. 



All figured specimens belong to the U. S. National Museum. The 

 technical work in connection with this paper has been carried out by 

 members of the staff of the Swedish Museum of Natural History in 

 Stockholm. 



Acknowledgments: For their generosity in pla,cing Ordovician 

 vertebrate material from a series of localities in North America at 

 my disposal, I wish to express my indebtedness to Drs. P. E. Cloud, 

 Jr. and R. J. Ross, Jr., of the U. S. Geological Survey in Washington, 

 D. C, and Denver, Colo., respectively, and to Drs. A. K. Miller and 

 W. M. Furnish of the University of Iowa. My thanks also are due 

 to Dr. D. H. Dunkle of the U. S. National Museum for his kind 

 assistance in procuring the material and to Dr. Jean M. Berdan of 

 the U. S. Geological Survey", Washington, D. C, for her information 

 concerning the stratigraphy at the type locality of Pycnaspis siilen- 

 dens, new genus, new species. 



Subclass PTERASPIDOMORPHI 



Superorder HETEROSTRACI 



Order ASTRASPIDA ' 

 PycnaspiSf new genus 



Diagnosis: A genus of the Astraspida of large size. Plates of the 

 exoskeleton thick. Ornamentation in early stages of growth con- 

 sisting of a shagreen of small tubercles exhibiting deep, radiating 

 grooves on their crowns; in succeeding stages of growth consisting of 

 stout, mushroom-like tubercles, smooth on top and ranging in outline 

 from round to elongated or somewhat kidney-shaped. All tubercles 

 consisting of an external layer of dentinous tissue and a basal portion 

 of bone (aspidm) surrounding a "pulp" cavity. In tubercles belong- 

 ing to early stages of growth the dentinous tissue is penetrated through- 

 out its height by very fine canals originally housing cell-processes, 

 and the aspidin is devoid of vascular canals issuing from the upper 

 part of the "pulp" cavity. In tubercles of succeeding stages of growth 

 the dentinous tissue in its basal part contains short, stemJike dentinal 

 tubes which, in a superficial direction, rapidly subdivide into very fine 

 ramifications, and the basal, bony portion is pierced by narrow, 

 straight vascular canals radiating upwards and outwards from the 

 roof of tlie "pulp" cavity and extending to about the boundary between 

 the aspidin and the dentinous tissue. 



> New rank, from Astraspldae Eastman, 1917. Synonym: Astraspiformes Berg, 1940. 



