26 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. _May 2, 



layers rest. This is shown in fig. 4 ; in which we see at /. f, f, the 

 three outer investing layers ; at ff, g, the two spaces interreniiig be- 

 tween them, which are prolonged from the marginal chambers ; and 

 at h, h, h. the vertical partitions, prolonged from the marginal septa, 

 by which these successive layers are supported one upon another. In 

 general these partitions tend with gi'eater or less regularity towards 

 the centre of the disk ; but this is not the case in X. Icevigata, for 

 here they run a sinuous and inconstant course, sometimes inoscula- 

 ting, and again diverging, so as to divide the whole surface of the 

 disk into a number of irregular areae. The origin of these partitions 

 from the septa of the marginal chambers, and the character of their 

 subsequent course, are seen in fig. 3 : at the left hand are shown 

 portions of three wh^orls which are laid open through the medial 

 plane ; whilst on the right are seen the investing portions of two of 

 these, with the broken edges of the partitions formed by prolonga- 

 tions of the marginal septa. A larger portion of one whorl, showing 

 the continuity of the septa and chambers in its marginal and inresf- 

 ing portions, is shown in fig. 6. As the course of these partitions is 

 a very sti-ongly-marked feature in the structure of Xummulites, and 

 presents many very obvious differences, I am inclined to think that 

 from such differences valuable specific characters may be obtained. 



I have now to describe one of the most curious, and until recently 

 unsuspected features in the structure of Xummulites; — I allude to the 

 existence of a series of perforations of considerable size, which pass 

 directly downwards from the exterior, through the supei^posed in- 

 vesring layers of the successive whorls, however numerous, unril they 

 reach the roof and floor of the chambers of the central plane, which 

 they do not penetrate. Various observers had Ttia&ikedi panctations 

 on the surface of certain species of Xummuhtes ; these being some- 

 times elevations and sometimes depressions. I believe that MM. 

 Joly and Leynierie were the first to sm-mise that these punctarions 

 are in reality the mouths of passages, which are blocked up by the 

 infiltration of mineral matter subsequently to the death of the animal, 

 and that their occasional projection from the siu*face is to be attri- 

 buted merely to the superior hardness of the mauer that fills them 

 (which forms the •'columns' or pillars of Mr. Sowerby, loc. cif.), 

 and to the shght abrasion of the shell around them. Guided by the 

 analogous characters presented by the indubitable perforations in the 

 shells of fossil Terebratulfe, I had myself anived at the same conclu- 

 sion, before I became acquainted with the researches of MM. Joly 

 and Leymerie .; having found that the substance with wliich these 

 perforations and passages are filled, evidently differs altogether from 

 the texture of the shell itself, and presents all the characters of a 

 calcareous infiltration. In fig. 1 is represented a thin section taken 

 nearly parallel to the surface, so as to pass nearly in the plane of 

 some of the investing layers ; the dark portion represents the shelly 

 texture already described ; the irregular light spaces on the middle 

 and left of the figure, ai"e the lacmite filled with calcareous infiltra- 

 tion ; whilst at the right, some of the chambers of the medial plane 

 are laid open. These passages may be frequently seen, in a piece like 



