EOCENE TEREBRATULA 
23 
(3) Relative convexity of the valves, 
(4) Character of the umbonal region. 
(5) Character of muscle scars and their platform. 
(6) Character of beak and pedicle opening. 
(7) Migration of the delthyrium backward. 
In those shells from the Eocene beds which show a slight tend¬ 
ency to plication, the character of the same is different from that 
of the early forms. In the Rancocas forms the lobing begins on 
the region of greatest convexity and proceeds anteriorly, produc¬ 
ing a groove or depressed area in the dorsal valve, thus giving 
rise in the ventral valve to a concave area to correspond with the 
convex area of the dorsal valve and two folds on either side of 
the convex area. The lines of growth are regularly curved on the 
Eocene form due to the absence of the plications. This loss of 
plication brings about another change, namely, in this region there 
is a slightly greater width of the valves. 
The area on either side of the crura is wider in the Eocene 
species, though this does not have as much significance as the 
preceding feature. There seems to be a tendency for the crura 
to come closer together and this is evidently the result of some 
morphological change in the internal parts and most surely has 
some weight in the way of morphological peculiarity. 
Terebratula marylandica shows greater convexity of the valves 
in all adult forms. It is not brought out so much by sketches as 
when actual specimens are compared. Both of the valves in T. 
harlani are flattened at the points where thickest, while the con¬ 
vexity is greatest at these points in T. marylandica ; which the cen¬ 
ter of greatest convexity is more anterior in T. harlani than in T. 
marylandica. The umbo in the new species is also more convex 
posteriorly. 
The high platform processes on either side of the internal groove 
is a striking difference distinguishing the Eocene and Cretacic 
forms. This process is very massive and stands 8 to 10 mm. 
above the groove. Its surface is undulating. In T. harlani there 
.is no platform in the majority of cases and in those that do 
possess it, it rises only a millimeter or so in height and is very 
smooth and small. 
The combination of characters of Terebratula marylayidica 
make the form stand out as a distinct and separate species and 
