564 DR. CARPENTER'S RESEARCHES ON THE FORAMIN1FEUA. 



ing in Nummulite}, and that the chamber of the row abutting on the preceding whorl 

 is nearly always much larger than the rest, and gives origin to two or even three 

 chambers in the next row. Further, it is shown by vertical sections (Plate XXXI. 

 fig. 7), that the innermost chambers of the whorl are not only broader but thicker, 

 their upper and under walls diverging from each other where they are to be con- 

 tinued over the spire they invest. Hence it is pretty obvious, that this portion of the 

 whorl is that wherein the most active nutrition takes place ; and it is here that the 

 marked accession to the number of chambers occurs, which tends to carry the later 

 rows around the whole circumference of the disk. 



115. Each of the early turns of the spire not only surrounds, but completely invests 

 its predecessor; as is best shown by a vertical section, such as that represented in 

 fig. 7- The investing whorl does not, in the younger part of the spire, come into 

 immediate contact with the two surfaces of that which it includes, but is separated 

 from it by the prolongation of the chambers and of their septa, very much as in 

 ordinary NumrnuUtes. But between the later whorls, there are no such interspaces. 

 The successive layers come into absolute continuity with one another ; both the tubuli 

 and the cones of non-tubular substance being continued from each into the one 

 external to it. From the time that the rapid thinning-away and opening-out of the 

 spire commences, the investment of the previously-formed whorls seems to discon- 

 tinue. It is at the margin of each whorl, that we find the intermediate or additional 

 skeleton most remarkably developed ; and the canal-system sometimes forms quite a 

 network in its substance (fig. 11). 



116. General Summary. It is obvious, from the foregoing details, that the physio- 

 logical condition of each individual segment of the animal of Heterostegina must be 

 essentially the same as that of each segment of Cycloclypeus ; and that the only 

 difference in the condition of the two organisms arises out of the mode in which 

 these segments are increased in number. In Cycloclypeus, in which each, row of seg- 

 ments is (normally at least) a complete annulus, a new annulus is formed around its 

 predecessor by gemmation from its several segments along the entire circumference ; 

 and this mode of increase may be traced-back to the central cell, which buds-out 

 equally on all sides. But in Heterostegina, each row is limited by the breadth of the 

 spire; and while most of its chambers are formed by the like kind of gemination 

 from their predecessors, there is a special provision for an augmentation in the 

 number of chambers at the end of the row nearest the previous whorl. Tracing-back 

 the spire to its origin, we find that it commences in the one-sided gemmation of the 

 central cell, just as we found it to do in Orbiculina (* 85) and in those forms of 

 Orbitolites which have a spiral commencement (^J 54). Hence there is nothing but 

 the plan of increase, which separates Heterostegina from Cycloclypeus ; and their 

 relation is exactly the same as that of Orbiculina and Orbitolites. For in Heterostegina, 

 as in Orbiculina, the first-formed portion is a spire, of which each turn invests its 

 predecessors ; but after three or four turns have been made, the spire spreads-out, 



