A New Genus of Chamidcc. 23 



minute vertical prisms which are visible only under a lens. The 

 middle shell -layer reaches a maximum thickness in old shells of 

 nearly or quite 30"'"'. This layer is composed throughout of a 

 mass of very small, angular thin-walled cells. The cells have a 

 good degree of uniformity of size, their longitudinal arrangement 

 and angular shape being quite distinctly seen by side view of 

 weathered specimens. By verticil view they are seen to be of ir- 

 regular shape and arrangement, but concentric lines of accretion 

 are sometimes observable in cross sections of the valve. The 

 inner porcellanous layer is thin, and under a lens it is seen to con- 

 sist of numerous concentric laminae. 



The upper valve is broadly convex, becoming large and strong- 

 ly arched in old shells; beak prominent, broad, incurved and 

 usually directed a little forward, but sometimes it has a slight 

 backward inclination. Surface, when the outer prismatic layer is 

 present, having an approximately smooth aspect; but it is marked 

 by numerous lines of growth and multitude of very fine radiating 

 lines. Specimens having the prismatic layer exfoliated or 

 weathered off show the edges of the numerous close-set radiating 

 septa of the outer portion of the middle layer. These septa are 

 moderately deep, and are so joined together by their inner edges 

 in pairs and double pairs, that the innermost spaces between them 

 are wider than those which appear at the outer surface of the 

 layer. The inner portion of the middle layer has a structure es- 

 sentially the same as that which the whole of the middle layer of 

 the lower valve possesses, but at the thinnest portion of the upper 

 valve this cellular portion of the middle layer is obsolete or want- 

 ing especially in young shells. The fact that the specimens of this 

 collection are so frequently crushed by pressure shows that they 

 were not so strong as their great thickness would indicate. 



All the details of the hinge have not been clearly ascertained, 

 The principal tooth of the lower valve is known to have been 

 very large and prominent, filling a large part of the space upon 

 the posterior side of the septum of the upper valve. The anterior 

 tooth of the upper valve is not well defined from the cardinal end 

 of the septum, but the posterior tooth is narrow, prominent, com- 

 paratively long, and curved outward. 



Fully adult shells reached a length of more than 200""", and a 

 diameter at the free margins of 90""" or more. 



Upon a preliminary examination of the collections sent by Mr. 

 Orcutt, and the less perfect examples obtained by Dr. Becker, 

 owing to the multicellular structure of the shell, which was con- 

 spicuously seen, I regarded this form as belonging to the genus 

 Sphaerulites. But after a more careful study, and working a 

 separated upper valve out of its imbedding matrix, it was found 

 to possess the hinge-structure that has just been described. The 

 specific name is given in honor of Mr. C. R. Orcutt, who first 

 discovered it. (Museum No. 12,698.) 



