GENUS CARPENTEllIA. 1S7 



309. Internal Struefure. — 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. 10), 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. 10, e, e\ e") resemble the principal septa {cl, d}, d") 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. 7 ; 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 (./j/',/") 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 I, b, b, fig. 18), and 

 there marking out an areolation which corresponds to that of the external surface. The 

 areolaj 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 

 (fig. 1 2, b, b, fig. 1 6). The reason of this peculiarity in their distribution will be presently seen. 



310. 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 chamber as it is formed conceals, than (with Dr. Gray) that it closes, the aperture of the 

 preceding chamber ; 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 chambers ; and that through this canal the previously formed chambers retain 

 their original connexion with the exterior. The general disposition of the chambers around 

 the central canal is well shown by sections of the cone taken parallel to its base (fig. 11); 

 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 (as at b, c) with the vertical 

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



31 1. The foramina which pierce the outer wall of each chamber are of considerable size, 

 as compared with the minute tubuli characteristic of the Nummuliitida, and they are not nearly 

 so closely approximated ; in both respects they correspond closely with the foramina of the 

 ordinary BotalicB and Globigerina. In fig. 1 3 they are shown as they appear in a section 

 traversing the wall somewhat obliquely to its surface, whilst in fig. 18 thev 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 other; but at a,a, fig. 18, it is seen that in the neighbourhood of 

 the ridges which project from the inner wall into the cavity of the chamber, the tubes either 

 bend or incline themselves in such a manner, that, whilst their external orifices are pretty uni- 

 formly distributed (fig. 12, «), their w^/'cy/^a/ orifices do not show themselves upon the ridges, but 

 are crowded together along their bases (fig. 12, b, and fig. 16). The septa, whether j»nV««r^ 



