CLASS PELECYPODA 
27 
straight in the middle; anterior side short, rather rounded. Length about 
13 mm. (Conch. Iconica.) 
Described as Yoldia glacialis Gray. 
Type in British Museum. Type locality, Arctic. 
Range. Arctic Ocean, Point Barrow. Also Atlantic. 
Genus YOLDIA Moller, 1842. 
Shell oblong, slightly attenuated behind; compressed, gaping, smooth 
or obliquely sculptured, with a dark olive shining epidermis; external 
ligament slight; cartilage as in Leda; pallial sinus deep. (Tryon. S. S. 
Conch.) 
Type. Yoldia myalls Couthouy. 
Distribution. Arctic and Antarctic seas, Greenland, Massachu¬ 
setts, Brazil, Norway and United States. 
Range in time. Silurian and Pleistocene. 
Yoldia thraciaeformis Storer, 1838. 
Plate 5, fig. 1. 
Boston Jour. Nat. Hist., 2:122; text figure. 
N. testa ovato-oblonga, transversa, nigra, crassa; antice rotundata, 
postice subtruncata et compressa, umbonibus prominentibus; cardine 
fovea magna. (Storer.) 
Shell ovate, transverse, equivalve, inflated, gaping at both extremi¬ 
ties, with numerous, very distinct, concentric lines of increment, covered 
with a yellowish-green, polished epidermis in young specimens, concealed 
under a black pigment, which readily rubs off in the recent specimen, 
giving a sooty appearance to the fingers. In the adult shell the epidermis 
is rather a dirty brown. Beaks slightly prominent over hinge margin. 
An obtuse angle, more elevated and wider at its lower half, runs obliquely 
from the umbones to the posterior base of the shell, serving as a boundary 
to the anterior inflated portion. Posterior portion of the shell much com¬ 
pressed, its epidermis is of a lighter color, and the striae of increase are 
much more apparent, than upon the anterior portion. Anterior margin 
rounded; posterior somewhat truncated. Within perlaceous. Teeth, 
numerous and peculiar; those contiguous to hinge, small, those farther 
removed from fosset, very strong, sharp, angulated, higher than wide; 
the teeth of one valve shutting very closely into the excavated teeth of the 
opposite valve, form a very powerful hinge. Length, 57; height, 35 mm. 
(Storer.) Described as Nulula thraciaeformis. 
Type in British Museum. Type locality, off Point Race, Atlantic. 
Range. Arctic to Oregon and Puget Sound. Also Atlantic. 
