NEW OSTRACODERM— 0RVIG 7 



Type species: Pycnaspis splendens, new species. 



Geological occurrence: Upper Ordovician: Upper part of the 

 Bighorn formation, eastern slope of the Bighorn Mountains, Wyoming. 

 Middle Ordovician: Whitewood formation, "Icebox" shale, Black 

 Hills Region of South Dakota; Winnipeg formation of Williston 

 Basin, Montana. 



Remarks: The order Astraspida, comprising the genera Palaeodus 

 Rohon, Astraspis Walcott, and Pycnaspis, new genus, has been defined 

 in another connection (Prvig, in MS., b). The diagnosis of Pycnaspis 

 given here lists mainly such characters by which this genus is dis- 

 tinguishable from Astraspis. 



The generic name is derived from the Greek tvkvos, thick, closely 

 set, referring to the shape and distribution of the tubercles of the 

 exoskeleton; and aams, shield. 



Pycnaspis splendens, new species 



Diagnosis: Same as for genus (single species). 



Holotype: USNM 21333; detached exoskeletal plate, showing 

 typical ornamentation (pi. 1, figs. 1, 2). 



Locality and geological horizon: The holotype and the other 

 material of Pycnaspis splendens, new genus, new species, dealt with 

 here were collected in 1951 by Dr. Jean M. Berdan, of the U. S. 

 Geological Survey, at the South Fork of Rock Creek, center, N}^NE}^ 

 SWKSW}^, sec. 25, T. 52 N., R. 84 W., Johnson County, Wyo. 

 (locahty number USGS 1362-CO; see fig. 1 and Darton, 1906a, p. 28). 

 According to personal communication by Dr. Berdan, there follows, 

 at this locality, on top of the massive Bighorn dolomite : (a) red shales 

 ("Red Bighorn") containing a rich fauna of Upper Ordovician in- 

 vertebrate fossils which seems to correlate with the Maquoketa of 

 Iowa, including, e. g., Lepidocyclus perlamellosus (Whitfield), L. cf. 

 rectangularis Wang, Sceptropora facula Uh'ich, and Streptelasma trilo- 

 hatum Whiteaves (cf. also the fauna Ust in Darton, 1906a, p. 28); 

 (b) red and green mottled calcareous siltstones and sandstones; 

 and (c) an upper sandy layer with numerous Ostracoderm remains. 

 The latter is directly overlaid by the Lovver Caiboniferous (Mis- 

 sissippian) Madison formation. By weathering, the fossiliferous shales 

 above the Bighorn Dolomite disintegrate into soil, and in this soil 

 the vertebrate material was found. Since, however, the plates and 

 scales always are associated with remnants of a coarse-grained sand- 

 stone, there is every reason to believe that they did not originally 

 belong to the red shales but are, in fact, derived from the beds above, 

 in all probability from the topmost sandy layer underneath the Madi- 

 son Ijimestone or, in other words, from the very youngest part of the 

 Bighorn formation exposed in the Rock Creek section. 



