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PALAEONTOLOGY OF NEW-YORK. 
specimens, or in the condition of casts. All the specimens which have the two 
valves attached are more or less crushed, as shown in figure 3. 
The specimens figured illustrate the usual condition of the species. 
Figures 1 & 2 are casts of a dorsal and a ventral valve of small size. 
Figure 3 is a dorsal view of a specimen having the shell crushed. 
Figure 4 is a cast of a large dorsal valve. 
Figures 5 & 6 show the area and interior of the ventral valve. 
Figure 7 is the exterior of a large ventral valve preserving the shell, and showing a remark¬ 
ably wide subangular sinus, with obscure plications on the sides of the shell. 
Figures 8-11 show the characters of the muscular impression. 
Geological formation and localities. In the shale of the Portage group, near 
Ithaca; and in a shaly sandstone of the same age, near Cqrtlandville, Cortland 
county, New-York. 
Spirifera mesacostalls. 
PLATE XL. 
Delthyris mesacostalis : Hall, Report of the Fourth Geol. District, p. 269, f. 9. 1843. 
Delthyris acuminata : Id. Ib. p. 270, f. 5. 
Hot Delthyris acuminata, Conrad, cited on page 198 of this volume. 
Shell variable in form, more or less gibbous, semioval or subquadrate; 
cardinal extremities usually a little salient : surface plicated. Tile 
width varies from once and a half to three times the length. 
Ventral valve ranging from semielliptical and moderately convex, to 
subquadrate and ventricose. In the less gibbous and more extended 
forms the convexity is pretty regular in the upper part of the shell, 
curving more abruptly towards the front, and nearly flat towards the 
cardinal extremities. The beak is small, and the upper part only ab¬ 
ruptly incurved over the area, which is of moderate height : the sinus 
is angular, varying in depth, and marked by a single defined fold in 
the bottom, which is usually distinctly angular, but rarely rounded or 
depressed. The fissure is higher than wide, and usually not closed by 
a pseudo-deltidium. 
Dorsal valve very gibbous in the middle and pretty regularly convex 
from beak to base, depressed towards the cardinal extremities, and 
