346 NORTH AMERICAN INDEX FOSSILS. 



27. A. abnormis Ulrich. {¥ig. 1660, h-j.) Siluric. 



With two thickened, elongate, rounded marginal nodes besides 

 the spine. 



Niagaran (Rochester shale) of New York, etc. 



Fig. 1659, ^<f/^w^«<^ 5/>/;z(9j^ (26), right valves, much enlarged. (After Hall.) 



28. A. marginata Ulrich. (Fig. 1660, ^.) Devonic. 



l^\k.t A. spinosa,h\xt with narrower border and much larger spine. 

 Hamilton of New York. 



XIII. Ulrichia Jones. 



Differs from Primitia in having a well developed node on each 

 side of the sulcus, which in this case is scarcely impressed. Geno- 

 type U. conradi Jones. About a dozen species. Ordovicic- 

 Mississippic. 



29. U. nodosa Ulrich. (Fig. 1658, 0, 0'.) Ordovicic. 

 Dorsal nodes very unequal, the anterior much the largest; two 



others near posteroventral margin. 



Lower Cincinnati group of Cincinnati region. 



30. U. emarginata Ulrich. (Fig. 1658, />,/?', g.) Mississippic. 

 Dorsal nodes nearly equal; other nodes nearer center than in 



preceding ; hinge line shorter. 

 Chester shales of Kentucky. 



XIV. Halliella Ulrich. 



Similar to Primitia, but with a larger sulcus, narrow at the 

 dorsal edge, and widening as it extends downward. 

 Ordovicic-Devonic. 



31. H. labiosa Ulrich. (Fig. 1658, r,/.) Ordovicic. 

 Nearly semicircular; hinge line forming nearly the greatest 



length ; sulcus narrow, short ; surface finely pitted. 

 Trenton of Minnesota, etc. 



