48 PALAEONTOLOGY OF KENTUCKY. 



becomes the more gibbous. Ventral valve moderately convex in young or 

 medium sized specimens, and gibbous in the upper part of old ones ; the 

 anterior portion depressed, and marked by a broad, undefined sinus ; beak 

 short, acute, closely incurved over the umbo of the opposite valve, area small, 

 short and sharply denned. Dorsal valve gibbous, and in old individuals the 

 umbo projects beyond the beak of the ventral valve, with the apex incurved 

 beneath the beak of the latter ; central portion of the valve toward the front 

 more elevated, and sometimes presenting a broad, undefined mesial fold. Sur- 

 face plications abruptly elevated, rounded, angular or sub-angular, becoming 

 depressed and sometimes obsolete on the cardinal slopes, usually simple, en- 

 larging toward the front of the shell, rarely bifurcating or intercalating in a 

 remarkable manner on the sides, where the folds bend abruptly outwards to 

 the cardino-lateral margins ; plications crossed by arching imbricating striae 

 or lines of growth, which are sometimes very conspicuous. The dimensions 

 of this species are very variable in different individuals. The figures on plate 

 XXXII. show the average size of Louisville specimens. 



Formation and. Locality. Occurs in the Niagara strata in the quarries east of the city of Louis- 

 ville, Ky., in fine, well preserved specimens, which are, however, rather rare, and which never attain the 

 size of individuals from "Waldron. To avoid mistakes, I will here draw the attention of the students of 

 palaeontology to the fact that in this species the valves are easily confounded, inasmuch as the dorsal one 

 is larger than the other, and its beak generally more prominent than the one of the ventral valve. 



Genus Camarella. Bnimgi. 



Camerella, Billings. Can. Nat. and Geol., Vol. 4 185.9. 

 Etymology: Kamara, arching chamber; ella, diminutive. 



This genus was established by Mr. Billings in 1859, but I have never been 

 able to see the description of his genus, which he should have repeated in his 

 "Palaeozoic Fossils" of the Geology of Canada, Volume I., 1861 to 1865, where 

 he describes eight different species of this genus. 



i The different shells which he places in his new genus have the following 

 characteristics : Shells sub-circular or ovate, unequivalve but equilateral ; 

 both valves more or less convex, having their greatest width below the middle 

 of the length, usually close to the base or front. The surface is generally 

 plicated but also smooth, and only marked by concentric lines of growth. The 

 internal characters of this genus are not known. 



Camerella congesta. CONRAD. 



Atrypa congesta, Conrad. Jour. Acad. Nat. Sci. Phil., Vol. 8 1842. 

 Atrypa congesta, Hall. Geol. Rep. 4th Dist. N. Y., p. 711343. 

 Atrypa congesta, Hall. Pal. N. Y., Vol. II., p. 671852. 



Shell sub-orbicular, gibbous, often more or less ovate, Ventral valve much 



