86 PALEONTOLOGY OF KENTUCKY. 



beak arching over the area ; dental lamellae strong, extending in short, obtuse 

 teeth. The impressions of the cardinal muscles form two semi-oval pits near 

 the middle of the concavity between the beak and base of the shell. Surface 

 very finely cancellated by obscure, radiating striae and fine concentric lines of 

 growth. Shell-structure fibrous or fibro-punctate ; luster pearly. The typical 

 species is Ambocoelia umbonata of Conrad. 



Ambocoelia umbonata. CONRAD. 



Plate XVII., figures 26 and 26. 



Orthis umbonata, Conrad. Jour. Acad. Nat. Sci., Phil., Vol. 8. 

 Orthis umbonata (Conrad), Hall. 10th Rep. on State Cab., p. 1671857. 

 Ambocoelia umbonata, Hall. 13th Kegent's Rep., p. 71 1860. 

 Orthis nucleus, Hall. Geol. Rep. 4th disk, pp. 180 and 181. 

 Ambocoelia nucleus, Hall. 13th Regent's Rep., p. 71. 

 Ambocoelia umbonata, Hall. Pal. N. Y., Vol. 4, p. 2591867. 



Shell very small, plano-convex, sub-circular ; length and width in our speci- 

 mens about equal ; hinge-line equal to the greatest width of the shell ; cardinal 

 extremities either obtuse-angular or rounded. 



Ventral valve gibbous, with the umbo extremely elevated, and a compara- 

 tively large incurved beak ; mesial sinus distinct, extending from the beak to 

 the front; it is shallow and rounded at the bottom ; its margins are also 

 rounded and not defined ; the cardinal area is rather large in proportion to the 

 small size of the shell ; its margins are sharply defined ; it is strongly concave, 

 and has a large triangular fissure in its center ; the beak is prominent and 

 strongly incurved into the upper portion of the foramen. 



Dorsal valve semi-elliptical, depressed convex in the upper central portions, 

 concave below the middle and at the sides ; apex scarcely elevated above the 

 hinge-line ; area very narrow and almost linear ; there is no indication of any 

 kind of a mesial fold. 



Surface marked by very fine radiating and concentric striae, the latter some- 

 times becoming crowded, lamellose and imbricating towards the front of the 

 valves. In the shells of our rocks these surf ace- markings are not visible, they 

 became obliterated by the silicification of the specimens. 



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

 county, Ky., and is mainly found in some washes east of the city of Louisville, where it occurs in great 

 abundance, but of considerably smaller size than eastern specimens of the same species. 



