44 PALAEONTOLOGY OF NEW-YORK. 



area inclined to the area of the opposite valve, and about one-half or 

 two-thirds as wide ; dorsal beak full, broad and rounded or obtusely 

 pointed, extending a little beyond and slightly incurved over the area. 

 Ventral valve less gibbous than the dorsal, very prominent on the 

 umbo, sloping very abruptly to the cardinal angles and more gently 

 towards the lateral and baso-lateral margins, flattened in the centre ; 

 the lower half marked by a gradually increasing, broad, and undefined 

 sinus, which gives a strongly curved outline to the front margin : beak 

 more acutely pointed and incurved ; area elevated and incurved, its 

 length equalling or greater than half the width of the shell. The beaks 

 of the two valves are approximate. 

 Surface marked by fine unequal striae which increase by interstitial 

 additions, and are crossed by fine equal concentric striae, and, at un- 

 equal intervals, by stronger imbricating lamellose lines of growth. In 

 certain conditions of the surface, the striae show tubular openings at 

 the concentric lamellose lines. There are from eight to ten of the larger 

 striae in the space of two lines ; and by counting the finer intermediate 

 striae, there are sometimes from thirteen to eighteen in the same space. 

 The cast of the dorsal valve shows a subquadrate or quadrate-ovate 

 muscular scar, with a deeply marked border and longitudinal median 

 groove :• a groove passes from the centre of the median line obliquely to 

 the lower lateral angles, dividing the adductor muscular scar. The va- 

 scular impressions radiate from the muscular impression in broad bands, 

 dividing several times before reaching the margin of the shell. 



This species approaches so closely the O. multistriata of the Upper Pentamerus 

 limestone of the Lower Helderberg group, that it is very difficult to distinguish 

 them, the form and surface charactess being generally almost the same. Some- 

 times, however, the ventral beak and area in this species is a little more arcuate 

 than in O., multistriata, and the dorsal valve is usually more gibbous, while in- 

 ternally they present well-marked differences. In this shell, the vascular im- 

 pressions bifiircate once, twice, or even three times, before reaching the border ; 

 wliilc in 0. multistriata, they pass down the front without bifurcation. 



Tlic Orthis TULLiENsis of the TuUy limestone is another form very difficult to 

 be distinguished from this one ; but it is generally more gibbous, and presents 

 internal differences ; the divisions of the vascular impressions pass down the 



