6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



Superfamily DALMANELLACEA Schuchert and LeVene, 1929 



Genus DICOELOSIA King, 1850 



Dicoelosia jonesridgensis Ross and Dutro n. sp. 



Plate 3, figures 1-5 



Description. — Shell concavo-convex, deeply emarginate giving ex- 

 terior the general shape of a maple seed (double samara) ; length 

 approximately equal to width which is greatest near antero-lateral 

 tips of the extended wings ; hingeline short, about half width of shell ; 

 area triangular and apsaclinal; valves weakly costellate, about 3 

 costellae in 0.5 mm, with a single rib more prominent on each wing; 

 commissure crenulated along sides of shell as well as in anterior 

 emargination. Pedicle valve strongly convex, sulcate, crudely pen- 

 tagonal in outline ; delthyrium open, apparently unmodified. Brachial 

 valve deeply concave ; anterior crenulation separated from visceral 

 cavity by flanges ; cardinal process arises from floor of simple noto- 

 thyrial cavity and has small expanded knob-like myophore ; brachio- 

 phores are slender rods lying along edges of visceral cavity and are 

 seemingly attached to shell for most of length with only the tips free. 



Discussion. — This species is represented by eight silicified speci- 

 mens, one of which shows the cardinalia quite well. Dicoelosia in' 

 dentus (Cooper) from the Gaspe is the only other known Ordovician 

 species in North America. The ornamentation of D. indentus is 

 coarser than that of the Alaskan form. D. alticavatus (Whittard and 

 Barker) as an emarginate anterior like the Alaskan species, but the 

 indentation is far narrower. In addition, the sides of the Silurian 

 shell are more nearly parallel and it is not as strongly concavo-convex 

 in lateral profile. D. jonesridgensis is more deeply emarginate than 

 either the Silurian species D. bilaba (Linne) and D. oklahomensis 

 Amsden or the Devonian D. varica (Conrad). Illustrations in Hall 

 and Clark (1892, pi. 10, fig. 18) show the brachiophores as flat blades 

 in the last species, dissimilar to those of D. jonesridgensis. However, 

 the cardinalia seem almost identical to those of D. oklahomensis as 

 illustrated by Amsden (1951, pi. 15, fig. 6). Brachiophores of Dicoe- 

 losia lata Wright from the Portrane limestone of Ireland (Wright, 

 1964, p. 226, pi. 9, figs. 9, 17) are wider and shorter than those in the 

 Alaskan species ; anterior emargination of the Irish species is less 

 pronounced. 



Holotype.— USNM 145347. 



Figured paratype.— USNM 145348. 



