96 PALEONTOLOGY OF NEW YORK. 



closed by a very convex cover. Viewed from above, the outline of the shell is 

 transversely oval, the posterior margin, the length of which has about one- 

 half the diameter of the shell, being straight. The ventral valve is elevated 

 and conical, and possesses an incurved and inflated umbo with no foramen. 

 Between the beak and the posterior margin is a sharply defined, elevated, tri- 

 angular area, which is divided by a low, convex ridge passing from the beak to 

 the margin. 



" The dorsal valve is convex, though much more depressed than the ventral, 

 and has a beak similar to that of the latter. Although this beak lies in the 

 same vertical line with that of the opposite valve, it rests directly upon the 

 posterior margin of the valve, and has no area. The surface of the valves is 

 smooth and covered only by fine concentric lines, which are continued without 

 interruption over the area of the ventral valve. ***** 



" Neither valve appears to possess any internal apophyses, at least the inter- 

 nal casts show no trace of them. The muscular impressions are unknown, the 

 casts showing only a small number of widely divergent radiating lines." (von 

 MOLLER, ut. at.) 



The similarity of this shell to Kutorgina in general form and surface-features, 

 is at once apparent. We meet, however, a conspicuous difference in the areal 

 ridge of Volborthia, a feature strongly suggestive of the " pseudo-deltidium " 

 of the articulate brachiopods. It is precisely similar to the areal ridge in 

 Iphidea, and the exact counterpart in form of the areal furrow in Acrotreta ; 

 its origin may be similarly explained, and a complete analogy may be found, in 

 this respect, in the genera with the foramen-scar ecurved as in Volborthia 

 and Iphidea, incurved as in Acrotreta and Conotreta, and open as in Kutor- 

 gina and ScHizoPHOLis. While we must admit the absence of an external fora- 

 minal aperture in the material so carefully studied and satisfectoril}' illustrated 

 by Professor von Mollef, we are led to surmise that this character, often so 

 obscure and difficult of detection in such fossils, will eventually be found. The 

 shells of Volborthia, like those of Kutorgina, are of very considerable size, an 

 example cited having a length of 14 mm., a width of 19 mm. and a height 

 of 16 mm. 



