72 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juiie 14, 



bright line, more or less continuous. The composition of the wall 

 was not satisfactorily ascertained ; but in roughly fractured sections, 

 which afforded occasionally a limited portion of a cavity unoccupied 

 by calcareous spar, minute foramina were visible. 



The nature of the intermediate structure must next be noticed. A 

 fine, bright line (fig. G), separating the walls of adjacent cavities 

 in the central area, has just been mentioned. It was not always 

 equally distinct, in consequence of structural interference ; and it 

 was frequently crossed by filaments or laminee ; occasionally also in 

 some sections by a bolder band, which had a symmetrical arrange- 

 ment, and formed an apparently continuous arch. In transverse sec- 

 tions of the central area, the fine line was less evident, yet detectable. 

 The passage of this delicate interspace from the axis to the outer 

 zone was often obscured, but cases of decided increase of breadth 

 occurred as well as of more prominent cross-filaments, which gave 

 the interval a cellular character. The equivalent space in the outer 

 zone was often narrow and clouded ; but the degree of opacity was 

 unequal, and where least, small, slightly translucent intervals were 

 separated by darker lines ; while in a transverse slice also with nearly 

 approximated abdominal cavities, simple or circular lines formed a 

 complete, intermediate, cellular structure. In some cases, the inter- 

 vals with a decided composition were clearly continuous with those 

 which separated the cavities in the central area. Where the space 

 between the visceral hollows was considerable (fig. 6), it consisted of 

 small, bright areas, sometimes oval, but more frequently of indefinite 

 form, and surrounded by opake lines ; occasionally these areas or 

 cells were arranged in definite rows, parallel to the exterior of the 

 branch, with a strong upper and lower continuous boundary ; the 

 latter structure however was not always conspicuous, and in many 

 cases inclosed more than one row of cells. The natural surface of a 

 specimen has been stated to exhibit, under certain conditions of 

 growth, a general network ; and its irregular meshes clearly occu- 

 pied the same position relative to the abdominal cavities as that of 

 the cells just mentioned : a branch likewise purposely worn down, as 

 nearly parallel to the exterior as possible, and without penetrating 

 beyond the outer zone, exhibited a similar network. The boundary 

 of the meshes or cells in both the natural and exposed surfaces was 

 continuous and thick, forming a perfectly surrounding wall, which 

 with the upper and lower laminse completely encompassed the areas. 

 It is inferred, from the cross-filaments in the fine intervals of the 

 axis, that those narrow spaces were cellular as well as the more am- 

 ple, and, therefore, that the occasional cloudiness before-mentioned 

 was due to the plane of intersection passing through or close to the 

 side-walls. With respect to the communications between the abdo- 

 minal cavities and the cells, and among the latter, where numerous, 

 it is sufiicient to allude to the microscopic pores, already mentioned, 

 in the periphery of the visceral hollows, and to state, that similar 

 foramina, or bright specks representatives of them, w'ere noticed in 

 the opake interspaces. 



The concentric layers shown in fractured or weathered specimens 



