172 PALEOZOIC PALEONTOLOGY. 
ward with a gentle curve to the postero-basal extremity, where it 
turns abruptly into the basal line; anterior end contracted in front 
of the beaks, of moderate length, rounded, most prominent a little 
above the middle. In a side view the beaks project very little, are 
compressed by a broad, shallow sulcus, which crosses the valves and 
occupies a large part of the anterior three-fifths of the shell; umbonal 
ridge rather distinct, extending from the beaks to the postero-basal 
extremity. In a cardinal view of casts of the interior, the only condi- 
tion in which the species has been noticed, the hinge-line is strongly 
depressed, lying at the bottom of a wide and deep channel, deepest 
between the rather widely-separated beaks and gradually shallowing 
posteriorly. Casts usually almost smooth, exhibiting only a small 
number of obscure, concentric furrows. One specimen preserves a 
small part of the shell, and this shows that near the dorsal edge the 
outer surface is marked with somewhat regular, raised lines, about 
six of them in 5 mm. The best-preserved casts exhibit in the posterior 
half of the mesial sulcus a number of obscure radii. Anterior mus- 
cular scar sharply defined at the inner side, rather small, broad-oval 
or circular, occupying the middle two-fourths of the upper half of 
the anterior end. Posterior impression somewhat larger than the 
anterior, subcircular, with a narrow prolongation extending forward 
nearly parallel with the posterior cardinal margin. Pallial line dis- 
tinct in the anterior half, consisting (on the casts) of a straight row 
of obscure pustules extending in a slightly-oblique direction from the 
base of the anterior adductor impression towards a point much nearer 
the ventral border.”—UIrich. f 
The dimensions of a nearly-complete internal cast of both valves 
are: length, 34 mm.; height, 11.5 mm.; thickness, 12 mm. 
Remarks.—This species was originally described from the Cin- 
cinnatian beds of the west, but the New Jersey specimens from the 
Trenton limestone seem to represent the same species. Like the 
western specimens, the New Jersey representatives are casts and the 
two valves occur together, but they do not grow to so large a size. 
The only difference noticeable in the New Jersey specimens which 
seems worthy of mention is the direction of the row of pustules form- 
ing the pallial line, which are more nearly parallel with the ventral 
margin of the shell. 
