FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 45 



hot occupying more than half the width of the valve ; foramen large, triangu- 

 lar, reaching to the beak, wider than high. Surface marked by sub-angular 

 plications, of which three occupy the mesial sinus, the central one being inter- 

 stitial, coming in below the umbo ; there are about ten plications on each side 

 of the mesial depression ; some of them originate by bifurcation or intercala- 

 tion. The radii are crossed by fine concentric lamellose lines of growth, 

 which gives them a sub-nodose appearance. This species has some resem- 

 blance to Orthis fissicosta, of the Hudson River group, but its cardinal ex- 

 tremities are more rounded and its cardinal area is not as high. 



Formation and Locality. Prof. Hall's specimen was found at Waldrori, Indiana, and that before 

 me I collected in the quarries east of the city of Louisville. It was imbedded in the white Niagara clay. 



Orthis vanuxemi. HALL. 



Plate XVI., figures 4, 5, 6, 12, 12o, 13 and 14. 

 Orthis vanuxemi. Hall. Tenth Hep. on St. Cab., p. 136 1857. 

 Orthis vanuxemi. Hall. Pal. N. Y., Vol. IV., p. 471867. 



Shell above medium size, sub-circular, or transversely sub-elliptical ; upper 

 part of the shell bi-convex ; lower or front part generally concavo-convex ; 

 shell strongly compressed ; hinge-line very short, little more than one-third 

 the width of the shell ; cardinal angles broadly rounded ; lateral margins 

 strongly curved, and the front in its central half inflected or emarginate. 

 Ventral valve mostly concave or sometimes flat, with the exception of the 

 umbonal region, which is slightly convex ; beak small, seldom extending 

 beyond that of the dorsal valve, but often even somewhat less elevated ; it 

 is pointed and gently arched, but not incurved ; cardinal area small, forming 

 a low triangle, which is divided in its center by a small triangular foramen, 

 partly filled by the cardinal process of the opposite valve. Dorsal valve 

 moderately, but even and regularly convex over the whole surface of the 

 valve, with the exception of the umbonal region, which is slightly more con- 

 vex ; the beak is scarcely distinct from the cardinal border and not incurved ; 

 cardinal process prominent and partly closing the ventral foramen ; cardinal 

 area small, about two-thirds as wide as the ventral, flat and inclined towards 

 that of the opposite valve. 



Surface of both valves ornamented by fine, closely arranged, radiating 

 tubular striae, which are perforate at intervals, increasing in number partly 

 by bifurcation and partly by intercalation, and crossed by very fine indistinct 

 concentric lines, and at greater intervals, by more distinct concentric, imbri- 

 cating lines of growth ; entire surface granulate or punctate under a magnifier. 

 Striae from twelve to sixteen in the space of two lines near the beak, and 

 from seven to nine in same space near margin. 



