Strophomena and Other Fossils 
25 
the hinge-area, and coalesce with the lateral, sharply raised border 
of the muscular area. This muscular area varies from rounded 
rhomboidal, to nearly circular in outline. The muscular area 
rises rather gradually toward its antero-lateral borders, but the 
slope from these borders toward the body cavity is abrupt. The 
chief characteristic of the muscular area is the presence of eight 
to ten rather indistinct radiating striae within the area of each of 
the diductor scars, separated by a low median elevation. This is 
the flabellate type of scar, in which the anterior and posterior 
divisions of the diductor scars are not distinctly differentiated. 
The entire interior of the body cavity, outside of the muscular 
area, is minutely granulose. In the case of very thin valves, in 
which the radiating striae ornamenting its exterior may be readily 
detected also on the interior surface, these granules are arranged 
along lines corresponding to the grooves between the exterior striae, 
but on the ordinary, thicker valves, these granules are arranged 
more irregularly. 
The antero-lateral raised borders of the muscular area, in case 
of the pedicel valve, approach anteriorly within about 3 mm. of 
each other and then curve sharply backward toward the lateral 
margins of the low median elevation which traverses the area 
from front to rear. This leaves an anterior gap, which is very 
characteristic of all species of Strophomena. The muscular area 
is surrounded on all sides, except at the gap, by a series of short 
ridges, arranged with their axes parallel to the radiating lines. 
These short ridges sometimes merge into longer, straight ridges, 
especially posteriorly, toward the hinge-area, but usually they are 
discrete. On each side of the gap, these short ridges cover an 
area extending about 2 mm. anterior to the raised border of the 
muscular area. Laterally, these ridges cover an area extending 
from 4 to 5 mm. from the lateral border of the muscular area, 
and they occupy the space directly posterior to that just de- 
scribed, as far as the hinge-area. 
The interior of the pedicel valve is thickened wdthin a short dis- 
tance of the anterior and lateral margins. Anteriorly, the maxi- 
mum thickening is about 4 or 5 mm. from the edge of the shell. 
Laterally, this thickening becomes less pronounced, and the maxi- 
mum thickening approaches to within 2 mm. of the lateral mar- 
gin, near the hinge-area. Occasionally this thickening equals 
fully IJ mm. anteriorly, but usually it is less than 1 mm., and may 
