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during a much greater augmentation of its length than of its diameter. 

 There is obviously a close physiological relationship between this 

 genus and the preceding, since the condition of each individual seg- 

 ment of the sarcode-body must be essentially the same in each ; and 

 it is merely in the mode in which these segments are multiplied, a 

 character which we have seen not to be constant in different parts 

 even of the same specimens of Orbitolites and Orbiculina, that it 

 differs from them. 



A marked contrast to Orbitolites and Orbiculina in all their phy- 

 siological characters, coexisting with an agreement in their respective 

 plans of growth, is presented by the genera Cycloclypeus and Hete- 

 rostegina ; the former of which, like Orbitolites, is cyclical from the 

 beginning, its chambers being formed in successive annuli round a 

 central cell ; whilst the latter, like Orbiculina, is spiral in the first 

 instance, but tends, as age advances, to assume the discoidal shape 

 and cyclical plan of growth. The genus Cycloclypeus is a new one, 

 founded by the author upon specimens dredged-up by Sir E. Belcher 

 off the coast of Borneo. These are the largest Foraminifera at pre- 

 sent known to exist ; the diameter of some of them being not less 

 than 2^ inches. The genus Heterostegina was formed by M. d'Or- 

 bigny ; but he seems only to have been acquainted with young spe- 

 cimens, and has altogether misapprehended its true characters and 

 relations. A fragment of the flattened spire of Heterostegina could 

 scarcely be distinguished from a marginal portion of the disk of Cy- 

 cloclypeus ; so close is the conformity between the two, as regards 

 the form and relations of the chambers, their mode of communica- 

 tion, and the structure of their shelly envelope. Each chamber, as 

 in Nummulites, has its own proper wall, so that the partition be- 

 tween the adjacent chambers, whether of the same row or of different 

 rows, is double ; and between its two lamellae there is interposed an 

 additional stratum of shell that belongs to neither. This additional 

 stratum is thin, in the septa dividing adjacent chambers of the same 

 row; biit it is much thicker, and forms a much more complete 

 separation, in the septa intervening between different rows. It is 

 traversed by a canal-system, analogous to that existing in Nummu- 

 lites ; which the author believes to be occupied in the living state by 

 threads of sarcode, and to be specially destined for the nutrition of 

 the 'intermediate skeleton' formed by the aggregate of these inter- 



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