FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 153 



Centronella glans-fagea, HALL. 



Plate XXXI., figures 14, 15, 16 and 17. 



Rhynchonella glans-fagea, Hall. Tenth Rep. on St. Cab., p. 125 1857. 

 Cenironella glans-fagea, Billings. Canadian Nat. and Geol., p. 131 1859. 

 Centronella glans-fagea, Hall. Sixteenth Rep. on St. Cab. 1863. 

 Centronella glans-fagea, Hall. Pal. H. Y., Vol. 4, p. 399 1867. 



Shell small, ovate or sub-triangular, concavo-convex, with rounded base and 

 very unequal valves. 



Ventral valve much larger than dorsal, greatly elevated in its center line 

 from beak to front, thus making the valve sub-carinate, or its transverse section 

 triangular. From the sub-carinate middle line it curves very abruptly to the 

 lateral margins, while longitudinally there is almost no curvature in the whole 

 valve except at the umbo; the beak is much extended beyond that of the 

 opposite valve, and arched dorsally far enough to extend to the level of dorsal 

 valve. 



Dorsal valve sub-angularly concave from beak to front, the depression cor- 

 responding to the angular elevation of the other valve ; the beak not incurving 

 into the opposite valve ; the mesial sinus widening rapidly towards the front, 

 extending in the basal third to the lateral margins. With the exception of 

 some concentric lines of growth, there are no other surface-markings. The 

 specimen illustrated is one of the largest found in Kentucky; it also shows the 

 usual proportion between width and length. Comparing our shells of this 

 species with Prof. Hall's figures, we find them differ from most of his forms in 

 size and shape ; still they have the specific aspect and characters. 



Formation and Locality. Found in the rotten hornstone of the Devonian group in Jefferson 

 county, Ky., and Clark couiity, Indiana, where it is, however, rarely met with, especially in well preserved 

 or even fair specimens. 



Genus Terebratula. Lihwya. 



Terebratula, Llhwyd. Lith. Brit. Ichn. 1696. 

 Etymology : diminutive of terebratus, perforated. 



Types : T. maxillata and T. vitrea. 



Shell minutely punctate, smooth and convex, usually round or oval. Ven- 

 tral valve with a prominent beak, which is truncated and perforated ; foramen 

 circular. 



Dorsal valve with a depressed umbo ; a prominent cardinal process between 

 the dental sockets ; deltidium of two pieces, frequently blended ; loop very 

 short, simple, attached by its crura to the hinge-plate, 



GEOL. SUK. 20 



