108 
PALEONTOLOGY OF NEW-YORK. 
Orthis plauoconvexa (n. %). 
Plate XII. Fig. 1-6. 
Shell plano-convex ; outline somewhat circular or transversely oval : 
length and breadth about as ten to twelve. Dorsal valve nearly flat, 
slightly prominent near the beak on either side of the faint central 
depression, and quite flat towards the margins. Ventral valve convex, 
sometimes scarcely subangular towards the beak; greatest convexity 
a little above the middle of the shell, and thence sloping uniformly to 
the lateral and basal margins : beak small, acute, incurved. Area 
linear, its length greater than half the width of the shell. Strige 
fasciculate, much curved upwards towards the cardinal and lateral 
margins. 
This species resembles 0. testudinciria in form; but the area is narrower, and the 
strirn are finer and less distinctly fasciculate : it is also less angular on the ventral 
valve, and the dorsal valve less sinuate. The cardinal process of the dorsal valve 
is much stronger, while the imprints of the adductor muscles are far less strongly 
marked. 
In the characters of surface strise, it is coarser than 0. elegantula, and the ventral 
valve less convex. It more nearly resembles the succeeding species (0. subcarinata ), 
but is more compressed, the carination of the one valve and the depression of the 
other being subdued, while the stria? are somewhat stronger, more distinctly fascicu¬ 
late, and more abruptly curving upwards towards the cardinal extremities. The 
interior also shows characters sufficient to distinguish it. In the dorsal valve, the 
lamella? bordering the muscular areas, which diverge abruptly, and then becoming 
almost obsolete, curve so as to enclose a broad oval space with a depressed line 
through the centre. This feature contrasts strongly with the prominent lamellae 
bounding the muscular impressions in the dorsal valve of 0. subcarinata. In the 
ventral valve the lamellae are broadly divergent, and, becoming gradually obsolete, 
are nearly lost, and leave scarcely an impression in the cast. The imprint of the 
adductor muscles forms a small scar towards the upper part of the vascular area, 
as in 0. oblata and shells of similar form. 
A single comparison of the interior structure will at once determine the question 
of identity or difference among these closely allied forms. 
