152 PALAEONTOLOGY OB 1 KENTUCKY. 



equal to greatest width of shell. Ventral valve convex or sub angular in the 

 middle, with beak more or less extended ; moderately incurved. Foramen 

 terminal, the lower side formed by two deltidial pieces. 



Dorsal valve flat or concave or depressed convex. A mesial fold and sinus 

 existing, but not often prominent. Structure of the shell lamellose or fibrous, 

 not punctate. Valves articulating by means of two strong teeth in the ventral, 

 inserted into the sockets in the dorsal valve, which are mainly excavated in 

 the base of a strong cardinal process. Teeth converging. Muscular impres- 

 sion marking a large ovate or flabelliform area, with a thin medium septum. 

 Adductor imprints small. 



Leptocoelia hemispherica. 



Plate XXXII., figures 21, 22 and 23, and 36 to 39. 

 Atrypa hemispheric^ Sovverby. Sil. Syst., p, 639 1839. 

 Atrypa hemispherica, Hall. Pal. N. Y., Vol. 2, p. 741852. 

 Atrypa hemispherica, Nicholson. Pal. of Ontario, p. 47 1875. 



Shell rather small ; hemispheric or sub-circular ; plicated. Hinge-line in 

 some specimens extended and straight ; in others short and deflected towards 

 the front. Both valves in the specimens before me somewhat convex. Surface 

 ornamented by about ten to twelve strong, rounded, simple radii, which in- 

 crease in strength and in size of their interspaces from apex to front. The 

 two shells illustrated, sub-figures 21 to 23, and sub-figures 36 to 39, agree in 

 every feature except in the size and direction of their cardinal lines. 



Formation and Locality. Found in the lower strata of the Niagara group, which may possibly 

 belong to the Clinton, in the quarries east of the city of Louisville, Ky., where it is, however, a rather rare 

 species, of which fair specimens are very seldom foumi. 



Genus Centronella. 



Centronella, Billings. Can. Nat. and Geol., Vol. 41859. 

 Etymology : a little point. 



Shells having the general form of Terebratula. Dorsal valve with a loop con- 

 sisting of two riband-like lamellae, which extend about one-half the length of 

 the shell. These lamellae at first curve gently outwards, and thence approach 

 each other gradually, until at their lower extremities they meet at an acute 

 angle ; then becoming united, they are deflected backwards towards the beak, 

 in what appears to be a thin, flat, vertical plate. Near their origin, each bears 

 upon the ventral side a single triangular crural process. This genus appears to 

 stand between Terebratula and Waldheimia. In the former the loop is short, 

 not exceeding greatly one-third the length of the shell, and not reflected. In 

 the latter it extends nearly to the front, and is reflected, but the laminae are 

 not united until they are folded back, (Billings.) 



