292 FAMILY NUMMULINIDA. 



convert the spiral into the cyclical plan of growth. But the differences between these two 

 types in regard to the texture of the shell, the mode of communication of the chambers and 

 charaberlets, and the presence or absence of an intermediate skeleton and canal system, are 

 such as most widely to separate them in any classification that is founded on characters of 

 true physiological value. The affinity of Ileferostegina to Cyclocli/peits again, is extremely 

 close ; the conformity of these two types, as regards every point of minute structure, being 

 complete ; and their difference being precisely that wiiich exists between OrhkuVma and 

 Orhifolites, — namely, that of the earlier plan of growth, which, in the former case, is spiral, 

 with a tendency to the cyclical, whilst in the latter it is cyclical from the beginning. To 

 Amphistegina, on the other hand, the relationship of this genus is very remote. 



492. GeograjjMcal and Geological DisfribuHon. — The genus Ilcferostegina is at present 

 known only as an inhabitant of tropical and sub-tropical seas ; having been found near 

 various parts of the East Indian, Australian, Polynesian, and West Indian shores, but not in 

 any colder latitude.— No other fossil examples of Hderosiegina have yet been met with, than 

 those which present themselves in the Vienna Tertiaries, in the Maltese deposit already 

 adverted to, and in a limestone apparently of Tertiary age on the south-east coast of 

 Arabia, in which Mr. Carter (xxiii a) has found HeterostegintB together with Orbitoides 

 and Cydoclypeus. 



Genus VI. — Cycloclypeus (Plate XIX, figs. 2 — 7). 



493. Historg. — This genus, first instituted by myself (xiv), is among the most interesting 

 of all the Foraminifera at present existing, on account botli of the large dimensions which it 

 sometimes attains, and of the light which its structure throws upon that of various fossil 

 types. The only specimens of it yet known were dredged by Sir Edward Belcher from a 

 considerable depth of water off the Coast of Borneo. Two of these, which are now in the 

 British Museum, are complete circular discs, measuring no less than 2\ inches in diameter ; 

 and by the kindness of Dr. J. E. Gray I have had the opportunity of making microscopic 

 sections of a fragment of a disk, which, when entire, must have nearly equalled these in size. 

 Smaller disks of various dimensions presented themselves in the same dredgings. 



494. External Characters. — The external aspect of these disks is sufficiently like that of 

 Orbifolites to prevent the two genera from being readily distinguished by a superficial 

 examination, especially when young specimens of Cgcloclgpeus are compared with OrbitoUtes 

 of the complex type ; since, on the two surfaces of the former (Plate XIX, fig. 2), there can 

 be distinguished concentric rings of oblong chambers, which are not at all unlike the 

 similarly disposed superficial cells of the latter. The peculiarly compact texture of the shell 

 of Cgcloclgpeus, however, gives to its surface a smooth and glistening appearance, which is very 

 different from that of Orbitoliles ; while the annular divisions and the radiating subdivisions 

 are indicated not so much by any inequahty of the surface (though they are usually somewhat 



