1*8 PALEONTOLOGY OF NEW-YORK. 



Oitliis planocoiive.va ( n. s.)- 



Plate XII. Fio. 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 subangidar 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. Striae 

 fasciculate, much curved upwards towards the cardinal and lateral 

 margins. 



This species resembles O. testudinaria in form ; but the area is narrower, and the 

 striae 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. eleganttda, and the ventral 

 valve less convex. It more nearly resembles the succeeding species (O. subcarinata), 

 but is more compressed, the carination of the one valve and the depression of the 

 other being subdued, while the strife 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 

 lamellae 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 featiire contrasts strongly with the prominent lamellae 

 bounding the muscular impressions in the dorsal valve of O. suhcarinata. 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 O. ohlata 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. 



