556 Forty-fifth Report on tre State Museum. 



to, or even below the general curvature of the internal surface. 

 A faint longitudinal septum extends forward from the platform. 



Brachial valve with low, sub-marginal, slightly-curved apex, 

 and short deltidium. Crescent conspicuously developed. Plat- 

 form as in the opposite valve, with the addition of a central 

 scar, which gives the entire area a tripartite appearance. 

 Median septum faint. 



Type, Rhinobolus Galtensis, Billings (sp.). 



Distribution. Upper Silurian ; Niagara and G-uelph faunas of 

 North America ; two species. 



Obolella, Billings. 1861. 

 (Plate 3, Figs. 5-7.) 

 Synonym; Dicellomus, Hall. 1871. 



Shell inarticulated, ovate or suborbicular, lenticular, smooth, 

 concentrically or radiately striated, sometimes reticulated by both 

 radiate and concentric striae. Yentral valve with solid beak and 

 a small, more or less distinctly grooved area. In the interior of 

 the ventral valve there are two elongated, sublinear or petaloid 

 muscular or vascular impressions, which extend forward from 

 near the cardinal scars, sometimes to points in front of the mid- 

 length of the shell. These are either straight or curved, parallel 

 with each other, or diverging toward the front. Between these, 

 at about the middle of the shell, is a pair of small impressions, 

 and close to the hinge-line a third pair, likewise small, and often 

 indistinct. There is also, at least in some species, a small pit near 

 the hinge-line, in which the groove of the area seems to terminate. 



The dorsal valve has a small, nearly flat hinge-facet ; the minute 

 beak is slightly incurved over the edge of the area. Beneath the 

 beak there is a small subangular ridge, on each side of which 

 there is a cardinal (?) scar. The elongate scars, which seem to 

 correspond to the lateral impressions in the ventral valve, are 

 here altogether in the upper half of the shell, diverging widely 

 in their extension forward, and are generally very slightly 

 impressed. In the cavity of the valve there is a low, rounded 

 median ridge, which extends from a point near the hinge-line 

 forward to a little below the mid-length of the valve. About the 

 middle of the shell there are two small sears. These are usually 



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