1849.] CARPENTER ON THE STRUCTURE OF ORBITOIDES. 33 



lites ; nor do they bear, like these, any numerical relation to the 

 chambers of the central plane. They are most numerous, however, 

 as in Nummulites, over the central portion of the disk, and gradually 

 diminish towards its margin. Their number seems to vary con- 

 siderably in different specimens whose diameter is about the same. 

 When a thin section is made through these layers, parallel to the 

 surface, the structure brought into view is very different from that 

 of any Nummulite ; for we see that it is made up of a number of 

 distinct flattened pieces of irregular form (resembling those of 

 O. Prattii, fig. 33), closely fitted together like the cells of the cuticle 

 of an apple-leaf, or the portions of a dissected map. Between the 

 successive layers thus formed, certain thin spaces are left, as shown 

 in fig. 20, which are circumscribed by the adherent margins of the 

 pieces between which they lie ; these spaces are not unfrequently 

 occupied, in the Alabama specimens, by an amorphous infiltration, 

 which renders them opake by transmitted light ; in the Indian speci- 

 mens they are filled with calcareous matter in a crystalline state, which 

 has also found its way into the chambers of the central layer. 



From the degree of alteration which all these specimens have un- 

 dergone in the process of fossilization, I am not able to give as com- 

 plete an account as I could wish of the minuter features of this struc- 

 ture. I am inclined to believe, however, that the several chambers 

 of the central layer communicate with each other by four or more 

 perforations through each septum, which some of my sections appear 

 to me to display, I cannot trace in this species any appearance of 

 passages by which the inhabitants of the chambers of the central 

 layer could have communicated with the world without ; but I shall 

 presently describe two methods of communication as discernible in 

 other species, and I have little doubt that one of them must have 

 existed here. 



The structure of Orhitoides Mantelli presents on an enlarged 

 scale all the essential features of that which I had previously made 

 out in the (so-called) Orbit olites Prattii of Biaritz. A section of 

 the latter fossil taken parallel to the surface, almost always brings 

 into view two distinct structures, as shown in fig. 32 ; that marked 

 a, a, and seen upon an enlarged scale in fig. 33, being evidently the 

 representative of the external layers of Orhitoides Mantelli ; whilst 

 that marked b, b, and seen as more highly magnified in fig. 34, 

 represents the chambered layer of the last-named species. That 

 such is the true account of it, is clearly indicated by the appearances 

 shown in vertical sections of these disks, a view of which, under a 

 low magnifying power, is given in fig. 35 ; whilst a small portion of 

 the section, more highly magnified, is shown in fig. 36. That both 

 structures should be almost invariably brought into view by a section 

 made parallel to the surface, is readily accounted for by the circum- 

 stance that the disks are seldom or never flat, so that, as they are 

 extremely thin, a plane section intended to pass through odc layer 

 must necessarily in some parts traverse the other. 



The chambered structure frequently presents considerable irregu- 

 larities, as seen at 5', fig. 32 ; I am not sure, however, that these are 



VOL. VI. PART I. D 



