566 DE. CAEPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



227. Internal Structure. On breaking-away a portion of the external wall of the 

 last-formed chamber, so as to lay open its interior (as shown in fig. 8), we find that its 

 cavity is closed-in on every side by its shelly walls, except where it has communications 

 (b, c) with the apical aperture ; and each principal chamber is partially subdivided by 

 a system of shelly septa, of which some are more and others less complete. The more 

 complete of these secondary septa (fig. 8, e, e\ e*) resemble the principal septa (d, d l , (?) 

 which separate the cavities of the chambers, in running from the base towards the apex 

 of the cone ; they divide the lower portion of each chamber into three, four, or more 

 digitations, which are sometimes marked by an external lobulation, as shown in fig. 5 ; 

 they stop short, however, about half or two-thirds of the way towards the apex, leaving 

 the upper third or half of the chamber undivided. Some of these septa do not reach 

 the opposite surface of the chamber; and the least complete (f,f l ,f) form a sort of 

 network of ridges slightly projecting from the inner surface of the outer wall into its 

 cavity (as shown in vertical section at b, b, b, fig. 15), and there marking out an areola- 

 tion which corresponds to that of the external surface. The areola3 of the internal 

 surface, however, are concave instead of convex ; and the punctations, which are wanting 

 on the ridges, are set more closely on the depressions between them. The reason of this 

 peculiarity in their distribution will be presently seen (^[229). 



228. The cavity of the last chamber communicates with the external orifice by a 

 passage of considerable size ; and the wall of this passage is distinctly continued as an 

 irregular ring around the apical aperture, so that this aperture may be considered in 

 one sense (as described by Dr. GRAY) to belong to the last chamber alone. But it would 

 be more correct to say that each cell as it is formed conceals, than (with Dr. GRAY) that 

 it closes, the aperture of the preceding cell ; for a careful examination shows that the 

 external aperture or vent is the termination of an irregular vertical canal, formed by the 

 superposition of the oral rings of successive cells; and that through this canal the 

 previously formed cells retain their original connexion with the exterior. In some of 

 Mr. PARKER'S specimens, the oral ring is extended upwards into a tube or siphon at 

 least equal in length to the radius of the cone. The general disposition of the chambers 

 around the central canal is well shown by sections of the cone taken parallel to its base 

 (fig. 7) ; such sections, however, may only bring into view the last or superficial whorl ; 

 and they will generally show only one or two chambers in communication with the 

 vertical canal, the communicating passage of each chamber being on a different plane. 



229. The foramina which pierce the outer wall of each chamber are of considerable 

 size, as compared with the minute tubuli of Cycloclypeus and Operculina (see - f[^[ 103, 

 154), and they are not nearly so closely approximated; -in both respects they correspond 

 closely with the foramina of the ordinary Rotalioe and Globigerince. In fig. 12 they are 

 shown as they appear in a section traversing the wall somewhat obliquely to its surface, 

 whilst in fig. 15 they are shown as they appear in vertical section ; and in each case they 

 are seen to present an annulated appearance, which is due to constrictions of the tubes 

 at tolerably regular intervals. These tubes generally pass direct from one surface to the 



