26 
PA LAE ONTOLOGY OF NEW YORK. 
deltidium, with diverging deltidial ridges, outside of which are the areal bor¬ 
ders, all marked with successive transverse growth-lines. This sheath appears 
to have been entirely enclosed by the shell about the umbo, but just at this 
point all the specimens before us are a little broken, and it is difficult to deter¬ 
mine whether or not its apex was extruded for a short distance. The slight 
gaping of the valves on the cardinal margin indicates that this was probable. 
Beneath the shelf thus formed lie the umbonal portions of the progressive 
muscular scars. The central and lateral scars are strongly developed, 
passing forward from the pedicle-shelf in three broad ridges, which 
terminate at the anterior margin of the platform. The crescentic mus¬ 
cular fulcrum is but faintly visible on this valve, though a broad flabellate 
scar, apparent on each side, probably comprises the terminal and posterior 
parietal impressions. The platform is comparatively thin and is solid, showing 
but slight excavation on its anterior wall. From its center a low median ridge 
extends about half-way over the anterior area of the valve; its margins are 
excavated by shallow furrows, which unite at the extremity of the ridge and 
extend to the anterior margin of the valve; on either side of this median fur¬ 
row is a broad, low elevation, the entire arrangement being equivalent to that 
seen in the pedicle-valve of Lingulops Whitfieldi, though the features are not so . 
conspicuously developed. 
In the opposite or brachial valve the crescentic ridge is very strongly 
developed, its central portion or crown being separate from the lateral 
portions. From its posterior angle the sides diverge for a short distance, 
ending abruptly; and from outside and behind these short arms, the ridge 
again appears, being broader and flatter than before, and sweeping in broad 
sub-marginal curves to the anterior edge of the platform. The external scars 
are more distinctly seen than in the pedicle-valve, but they are not very satis-, 
factorily defined. From the crown of the crescent, a low axial septum passes 
forward to the edge of the platform, dividing the central scar, on either side 
of which may be seen the lateral impressions. The platform is moderately 
high and its edge is a little incurved between each ante-lateral angle and the 
median septum. The latter feature is very strongly developed where it joins 
