22 
EOCENE TEREBRATULA 
is somewhat shallower and more concave. The foramen in T. harlani is 
proportionately larger than in T. marylandica. The beak is very much 
stouter in the Eocene form and thicker, the layers showing up well. On 
the ventral valve of T. Marylandica the cardinal area is distinctly diiferent 
from the corresponding area of T. harlani; this change being brought 
about by the deposition of; material anterior to the pedicle opening. This 
deposit is of sufficient size and area to deserve notice and is. quite promi¬ 
nent when a number of specimens' are compared. The deposit shows very 
fine, closely spaced lines of growth. 
Dorsal Valve: Convex, in some specimens more or less than the ventral 
valve, but usually the same; the curvature from the highest part very 
abrupt to the beak; uniformly curved and nowhere with valves showing 
any tendency toward bilobation; both valves show two calcareous layers, 
the outer one of a sheeted nature everywhere paralelling the surface of 
the valve, the inner one being of a prismatic nature; the outer layer is 
uniformly well) preserved. The curving of the beak is far more abrupt in 
the Ex)cene specimens than in the Cretacic examples. Most of the valves 
are more abruptly curved or compressed posteriorly, even curving slightly 
forward at the hinge margin. 
External Ornamentation: Lines are uniformly curved, that is, there are 
no major or minor lines. There are punctae at right angles to these curved 
lines, rarely visible to the naked eye. 
Internal Ornamentation: The dorsal valve has a relatively deep narrow 
groove extending over three-fourths the entire length of the valve, narrow 
near the crura, but widening considerably towards the anterior; on either 
side is a triangular muscle-scar of the adductor, diductor and pedicle 
muscles; in the median portion of the dorsal valve the groove is deepest 
at these platforms, and the groove dies out as the valve becomes thinner 
toward the edge; valve very massive towards the crura. The latter are 
flattened at their base, and the loops are not well preserved, although there 
is every reason to suppose that the brachidium was very similar to that of 
the true T. harlani. There is a wide area on either side of the crura; the 
distance across the valve from hinge to hinge is relatively narrower than 
in T. harlani. The ventral valve shows a groove running through the 
center, which widens towards the anterior; on either side of the groove 
are the thick, heavy platforms of the muscle, which are uneven on their 
surface; foramen possesses a wider diameter externally. 
For comparison the description of Morton,^ Whitfield,^ Weller,® 
Gardner,^ and Clark and Morton,® are not without interest. 
The distinguishing features of T. Marylandica: 
(1) Absence of plication, 
(2) Character of the hinge area, 
4 Am. Jour. Sci., (1), Vol. VI, p. 280, 1829. 
5 Mon. U. S. Geol. Surv., Vol. IX, Pt. vi, p. 6, 1885. 
6 Geol. Surv. New Jersey, Pal., V^ol. IV, p. 357, 1907. 
7 Maryland Geol. Surv., Vol. on Upper Cret., p. 734, 1916. 
8 Maryland Geol. Surv., Vol. on Eocene, p. 204, 1901. 
