38 
PALEONTOLOGY OP NEW-YORK. 
Orthis caiiiiatao 
PLATE VIII 
Orthis carinatd : Hall, Geol. Report Fourth District New-York, 1843, page 267, f. 1. 
Shell transverse, often much wider than long, subplano-convex : hinge- 
line nearly or quite equal to three-fourths the greatest width of the 
shell, and about equal to its length; cardinal extremities rounded. The 
dorsal valve is very convex or gibbous, with a deep rounded sinus in 
the median line, abruptly curving towards the cardinal line and some¬ 
what more gently to the front, depressed and scarcely flattened at the 
cardinal extremities. Ventral valve nearly flat, or rising from the base 
to the umbo with little convexity : median line carinate, with a strong 
and defined angular ridge, front abruptly sinuate; area comparatively 
narrow, a little curved near the beak : foramen wide. 
Surface marked by fine radiating striae, which increase by bifurcation 
and interstitial additions, and are strongly curved upwards to the 
hinge-line : texture punctate. The specimens are mainly casts, and the 
lines of growth are obscure. 
The cast of the dorsal valve shows a wide subquadrate muscular im¬ 
pression, divided by a rounded median ridge, and transversely by a low 
ridge on either side, after the manner of 0. elegantula and others of that 
type. The socket plates are strong and very divergent, and the cardinal 
process is apparently triplicate : the area is narrow and flat. 
The cast of the ventral valve is nearly flat on the sides, with a defined 
angular carina along the median line. The muscular impression is sub- 
quadrate, deeply bilobed below, and slightly lobed at the sides. The dental 
lamellae are strong and triangular. 
In specimens which have suffered no compression, the length is about 
three-fourths the width; but in some individuals, the width is nearly 
double the length. The specimens are usually more or less distorted by 
pressure, and are likewise mostly in the condition of casts; so that all 
the characters of the species cannot be given. The dorsal valve is more 
convex than others of this type of Orthis, and the median sinus more 
strongly developed. 
