NEW OSTRACODERM— 0RVIG 



17 



animal determinable with any degree of certainty. This specimen, 

 which is particularly large and well preserved, in all probabihty repre- 

 sents a branchio-cornual plate, unknown so far in any of the pre- 

 Silm'ian Heterostraci (pi. 2, figs. 4-7). A second specimen, which may 



o can d 



Figure S. — a, Eriptychiida gen. and sp. indet.: Ornamentation of a plate-fragment from 

 the Pycnaspis-htd^Ting beds of the Rock Creek section, in oblique lateral view (mainly 

 after USNM 21339, X36). b, c, Eriptychius sp., Winnipeg formation, Williston Basin, 

 Mont., sections of scales (from 0rvig, in MS, a): b. Vertical section showing the dentine 

 ridges of the external face (cut somewhat obliquely to their axes of length), the middle, 

 vascular layer, and the basal layer (section No. S 1438; X36); c, horizontal section of 

 the dentine ridges and the adjoining part of the middle, vascular layer (section No. S 

 1454; X36). Explanation of symbols: bal, basal layer of the scales; can.ascb, ascending 

 vascular canals piercing the basal layer; can.asc,, vascular canals in the upper part of 

 the middle layer of the scales, ascending towards the dentine ridges; can.d, short vascular 

 canals extending outwards in the basal part of the dentine ridges and opening to the ex- 

 terior at the basal circumference of those ridges; cati.l, longitudinal vascular canals in 

 the basal part of the dentine ridges; dt, dentinal tubes; mil, middle, vascular layer of the 

 scales; o.can.tf, the openings of vascular canals at the basal circumference of the dentine 

 ridges; sul, superficial layers of the scales. 



438551—58- 



