96 PALAEONTOLOGY OF KENTUCKY. 



Cyrtina hamiltoniae. HALL. 



Plate XIII., figures 4 to 12. 



Cyrtia hamiltonensis, Hall. Tenth Kep. on St. Cab., p. 1661857. 

 Cyrtia hamiltonensis, Billings. Dev. Foss. of Can. "West., p. 263 1861. 

 Compare Cyrtia acutirostra, Shumard, Geol. Kep. Missouri, part 1, p. 204 1854. 

 Cyrtina hamiltonensis, Nicholson. Pal. of Ontario, p. 83 1874. 

 Cyrtina hamiltoniae, Hall. 24th Rep. N. Y. St. Mus., p. 1981872. 



Shell small, more or less triangular, sub-pyramidal ; hinge-line equal to 

 greatest width of shell ; proportions of length, width and depth variable, but 

 frequently width is equal to length of ventral valve, and height of area is equal 

 to length of dorsal valve. Surface plicate. 



Ventral valve quadrilateral in outline, obliquely sub-pyramidal, most promi- 

 nent at beak, which is very variable in elevation, and straight or a little arched 

 over cardinal area, and not unfrequently attenuate and distorted or turned to 

 one side ; mesial sinus wide and strongly defined, rounded or sub-angular in 

 bottom ; area variable, large and elevated, plane or arcuate in different degrees, 

 with its lateral margins angular, distinctly striate in both directions ; fissure 

 narrow, closed by a convex, pseudo-deltidium, which is perforated above by 

 an oval foramen. Dorsal valve depressed convex, with a broad, more or less 

 prominent mesial fold, which is bounded by broader furrows than those be- 

 tween the plications, and is sometimes extremely elevated in front ; beak 

 scarcely rising above hinge-line ; area narrow, almost linear, but quite distinct. 



Surface marked by about six to eight (rarely one or two more) simple, 

 rounded plications on each side of mesial fold or sinus, and these are crossed 

 by very fine concentric lines of growth, which, at intervals, become crowded 

 and sub -imbricate, especially towards margins of shell. The finer surface- 

 marking is minutely granulose or papillose and shell structure distinctly punc- 

 tate. In some of the larger individuals there is an obscure elevation on each 

 slope of sinus, resembling an obsolete plication. The longitudinal medium 

 septum extends for more than half the length of the ventral valve, and is con- 

 tinued into the cavity beneath pseudo-deltidium. These features are shown 

 in casts and in transverse sections of valve. The dorsal valve shows a double 

 or bilobed cardinal process with the strong crural bases supporting spiral arms, 

 which are directed into the two compartments of the ventral valve, and, 

 making numerous turns, terminate in the rostral part of the shell. 



Formation and Locality. This species is found in the Hamilton group in different localities. At 

 Widder Station, in Canada West, it occurs in great abundance, and in most perfectly preserved specimens. 

 In the Corniferous group around the Falls of the Ohio it is also somewhat abundant in good but silici- 

 fied specimens. 



