FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 155 



Terebratula lincklaeni. HALL. 



Plate XVII., figures 22, 23 and 24. 



Terebratula lincklaeni, Hall. 13th Regent's Report 1860. 

 Terebratula lincklaeni, var., Hall. Pal. N. Y., Vol. 4, p. 4181867. 



Shell of medium size; sub-ovate or sub triangular, with broadly curved or 

 slightly truncated base ; moderately ventricose; length somewhat exceeding 

 width. 



Ventral valve larger, and slightly more ventricose than the dorsal ; greatest 

 convexity at the unibo, from where it slopes in a gentle regular curve to the 

 lateral and basal margins, but abruptly towards the cardinal margins, whose 

 borders are somewhat inflected ; the umbo is prominent, and the beak elevated 

 and incurved above the beak of the other valve ; at the front there is a slight 

 depression noticeable, but this feature is often wanting. 



Dorsal valve smaller, and less convex than the ventral ; greatest convexity 

 at the umbo, from where it slopes in an almost straight, line to the front, in a 

 gentle curve to the lateral margins and abruptly to the cardinal lines ; umbo 

 moderaetly prominent, and the beak strongly incurved under that of the 

 opposite valve. 



The surface is covered with concentric lines of growth, some of which are 

 more prominent, and divide the surface of some individuals into different 

 zones. No other surface-markings are visible on our specimens, which are all 

 in a silicified condition. 



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

 county, -Ky., and in Clark county, Ind.; it is of rather rare occurrence. This species resembles Ter.roemin- 

 geri, with which it is associated, but differs by having its greatest width near the base, while Ter.roemin- 

 geri has its maximum width above the middle, and the latter species is generally more gibbous and stronger 

 umbonated. 



Terebratula roemingeri. HALL. 



Plate XVI., figures 20, 21 and 22. 



Terebratula roemingeri, Hall. Sixteenth Rep. on St. Cab., p. 48 1863. 

 Terebratula roermngeri, Hall. Pal. N. Y., Vol. 4, p. 3891867. 



Shell ovate, more or less gibbous ; truncate or slightly sinuate in front. 



Ventral valve gibbous above the middle; umbo gibbous, inflated; beak promi- 

 nent, incurved over that of the opposite valve, and truncated by a round for- 

 amen, which is often mainly anterior to the apex, and completed on the lower 

 side by two deltidial plates ; cardinal slopes rounded, often depressed in the 

 middle towards the front. 



Dorsal valve extremely gibbous, little longer than wide ; the greatest con- 

 vexity at the middle or above. 



