FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 37 



incurved; cardinal area of about half the height of the ventral, and slightly 

 curved. Surface of both valves ornamented by distinct, rather prominent, 

 radiating ribs, of which from thirty-six to forty-four may be counted on the 

 margins of shell, most all of which extend from beak to front, Bifurcation 

 is very seldom noticed, and then only in old and very large specimens. With 

 the exception of a few imbricating lines of growth, no other surface-markings 

 are visible. This species resembles Orthis plicatella, but is more convex, and 

 has more numerous and smaller costae ; it is also less transverse, and its mesial 

 depression and elevation are more marked. 



Formation and Locality. Found at Frankfort, Ky., in the Trenton limestone, in great abundance, 

 and in excellently preserved specimens. 



Orthis elegant ula. DALMAN. 



Plate XXXII., figures 62 to 57. 



Orthis elegantula, Dalman. K. Vet. Acad. Handl., p. 117 1827. 

 Orthis canalis, Sowerby. Murch. Sil. Syst.. p. 631 1839. 

 Orthis canalis, Hall. Geol. Rep. 4th dist. N. Y., p. 1071843. 

 Orthis elegantula, Hall. Pal. N. Y., Vol. III., p. 2521852. 

 Orthis elegantula, Hall, llth Kep. of Indiana, p. 2851881. 



Shell small, semi- oval, plano-convex, sometimes even somewhat concavo- 

 convex ; length usually exceeding width. Hinge-line less than greatest width 

 of shell, cardinal extremities rounded. 



Ventral valve gibbous, greatest convexity about the middle of the valve, 

 from where it slopes in regular curves gently to front, more abruptly to lateral 

 margins, umbo prominent, beak narrow, elevated and incurved over cardinal 

 area, which latter is depressed triangular, not reaching to cardinal extremities; 

 foramen of medium size, its base to height as two to three ; in the center of 

 the valve is sometimes a slight linear elevation, which extends from beak to 

 front, visible ; lateral margins regularly but gently curved ; anterior margin 

 forms a regular semi-circle, seldom showing in its center a slight sinuosity. 

 Dorsal valve either flat or even a little concave, rarely being slightly convex, 

 often showing a gentle linear depression in the center line of the valve from 

 beak to front ; hinge-area narrow, almost linear ; beak small and incurved. 

 Surface marked by fine radiating striae, which increase towards the margins 

 by bifurcation ; the lateral ones are strongly curved outwards. This beautiful 

 species is closely allied to Orthis testudinaria, and, perhaps, more nearly to 

 O. parva, both of which are Lower Silurian fossils. In our specimens the 

 shell is generally more elongated, and the beak more extended, while the sur- 

 face is more finely striated. Orthis elegantula is easily distinguished. In 



