22 West American Scientist. 



layer of the upper valve has the same cellular structure as that 

 which has just been described for the lower valve, except that the 

 longitudinal arrangement of the cells is not so evident; and the 

 teeth of both valves are mainly or wholly composed of shell-sub- 

 stance which possesses this cellular structure. The outer portion 

 of the middle layer of the upper valve, however, has a structure 

 similar to that which is possessed by the upper valve of Plagiop- 

 tychus. It consists of numerous thin, close-set radiating vertical 

 plates or septa which extend from the beak to the free margin, 

 their outer edges abutting against the outer prismatic layer, and 

 at their inner edges they blend with the inner cellular portion of 

 the middle layer. 



This proposed new genus stands near to Plagioptychus, Ichthy- 

 osarcolithes, and Caprina, agreeing with them all in the general 

 character of the hinge and of the ligamental grooves, and in the 

 presence of an outer prismatic shell layer. It further agrees with 

 Plagiotychus in the structure of the outer portion of the middle 

 shell- layer of the upper valve; but it disagrees with that genus 

 in the multicellular structure of the thick middle layer of the 

 lower valve, and that of the inner portion of the middle layer of 

 the upper valve; those portions being porcellanous in Plagiotychus. 

 Comparing the shell-structure of Coralliochama with that of 

 Ichthyosarcolithes, the cells of the former are seen to be much more 

 uniform in size ; and the large so-called water-chambers in the 

 thick shellwalls of the latter are wanting in those of the former. 

 Also the slender radiating plates or septa of the outer portion of 

 the middle layer of both Coralliochama and Plagioptychus are ap- 

 parently wanting in Ichthyosarcolithes. At least such a structure 

 seems never to have been described. 



CAROLLIOCHAMA ORCUTTI (sp. nov.) 



Shell reaching a large size, growing both singly and in clusters, 

 one example in the collection showing a cluster of eight in- 

 dividuals. Lower valve irregularly subpyramidal, usually much 

 elongated and more or less distorted ; its free border transverse 

 or oblique, the posterior side in the latter case being the lower; 

 ligamental groove somewhat distinct; the space which was occu- 

 pied by the animal comparatively small, eccentric, the thinnest 

 portion of the shell-wall being at the part which corresponds to 

 the postero -basal part in ordinary bivalves. Surface, when the 

 outer prismatic layer is present, marked by fine longitudinal Hues, 

 and somewhat more distinct concentric lines of growth. The 

 coarser markings of the valve consist of a greater or less number 

 of concentric undulations, which are crossed by longitudinal cor- 

 rugations that are usually more conspicuous upon the side which 

 bears the hinge, especially the part adjacent to the hinge margin. 

 The prismatic layer is moderately thin, and is composed of 



