40§ PAL.'EONTOLOOY OF NEW- YORK. 



of concentric strioe or lines of growth are sometimes preserved. The 

 radiating strijB on the upper lateral portions of the shell are curved 

 towards the dorsal margin, gradually increasing in the curvature till 

 ( as shown in the cast of the dorsal valve ) the direction becomes re- 

 versed. 

 The interior of the dorsal valve shows a very subordinate cardinal process 

 and two strong lamellae or brachial processes, which are joined to the 

 shell by a ridge from the base on either side, and a well-defined 

 median septum extending from the base of the cardinal process : the 

 vascular impressions are not distinctly seen. The interior of the ventral 

 valve is marked by strong muscular imprints which occupy a large 

 cordiform space, reaching more than half and often two-thirds of the 

 length of the shell from beak to base : the central portions, or the 

 imprints of the adductor muscles, are often well defined. Surfaces of 

 muscular imprint marked by large radiating grooves and ridges in 

 old shells ; while in the young shells, these markings are much less 

 conspicuous or altogether wanting, and the outline limit of the mu- 

 scular impression itself is but faintly defined along its lower margin. 



This is the largest Orthis known to me in our rocks, and is quite equal in size 

 to any of the Carboniferous species which have fallen under my observation. It is 

 retidily distinguished by its large and strongly marked muscular impression, which, 

 however, presents a considerable variation in the degree of development dependent 

 on the different stages of growth, and in some measure apparently upon the nature 

 of the sediment in which it lived. 



PLATE LXXXIX. 



Fig. 1 a. A dorsal valve which retains a small portion of the shell. The direction of the striae 

 upon the hinge-margin is very well shown. 



Fig. 1 b. Profile view of the same. 



Fig. 2 a. A similar cast, showing the vascular impressions upon the surface of the more 



prominent portions, and likewise towards the anterior margin. 

 Fig. 2 b. Profile of the same from the cardinal side. 



Fig. 2 c. The interior of the upper part of the dorsal valve and brachial processes, as shown 

 in a mould from the preceding figure. 



Fig. 4. The cast of a shell of medium size, preserving the muscular impression in its usual 

 symmetrical form. 



Fig. 3, A very old specimen, in which the muscular impression is extremely elongated. 



