268 
PALAEONTOLOGY OF NEW-YORK. 
Cyrtina Iiainiltonensis. 
PLATES XXVII &. XLIV. 
Cyrtia hamiltonensis .- Hall. Tenth Report on the State Cabinet, p. 166. 1857. 
« << : Billings, Devonian Fossils of Canada West (Canadian Journal), p. 263. 1861. 
Compare Cyrtia acutirostra, Shumard, Geol. Report Missouri, Part i, pa. 204, pi. c, f. 3. 1854. 
Shell more or less triangular-subpyramidal; hinge-line equal to the 
greatest width of the shell; proportions of length, breadth and height 
variable, but frequently the width is equal to the length of the ventral 
valve, and the height of area is equal to the length of the dorsal valve : 
surface plicate. 
Ventral valve quadrilateral in outline, obliquely subpyramidal, most 
prominent at the beak, which is very variable in elevation and straight 
or a little arched over the area, and not unfrequently attenuate and 
distorted or turned to one side; mesial sinus wide and strongly defined, 
rounded or subangular in the bottom : area variable, large and elevated, 
plane or arcuate in different degrees with the lateral margins angular, 
distinctly striate in both directions; fissure narrow, closed by a con¬ 
vex pseudo-deltidium, which is perforated above by an oval or narrow¬ 
ly ovate foramen. 
Dorsal valve depressed-convex, with a broad more or less prominent 
mesial fold, which is bounded by broader furrows than those between 
the plications, and is sometimes extremely elevated in front; beak 
scarcely rising above the hinge-line : area narrow linear, but quite 
distinct. 
Surface marked by about six to eight (rarely one or two more) simple 
rounded plications on either side of the mesial fold and sinus, and these 
are crossed by very fine concentric lines of growth, which at intervals 
become crowded and subimbricate, especially towards the margins of 
older shells. The finer surface-marking is minutely grknulose or papil¬ 
lose, and the shell-structure distinctly punctate. In some of the larger 
individuals there is an obscure elevation on each slope of the sinus, 
resembling an obsolete plication. 
The longitudinal median septum extends for more than half the length 
of the ventral valve, and is continued into the cavity beneath the pseudo- 
