58 PALEONTOLOGY OP NEW- YORK. 



Oitlils carinata. 



PLATE Vin. 

 OrthU earinata : Hall, Geol. Report Fourth District New-York, 1848, 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 ptrong 

 and defined angular ridge, front abruptly sinuate ; area comparatively 

 narrow, a little ciu-ved 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 imjiression is sub- 

 quadrate, deeply bilobed below, and slightly lobed at the sides. The dental 

 lamellae are strong and triangular. 



In specimens which have sufiered no compression, the length is about 

 three-fourths the width ; but in some individuals, the width i§ 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. 



