FORAMINIFERA AND RADIOLARIA. 



99 



ing system of passages excavated in the marginal cord, are extremely 

 well seen in the 'internal cast' represented in Fig. 337. 



491. A very interesting modification of the Nummuline type is pre- 

 sented in the genus Heterostegina (Fig. 338), which bears a very strong 

 resemblance to Orbiculinam its plan of growth, whilst in every other 

 respect it is essentially different. If the principal chambers of an Oper- 

 culina were divided into chamberlets by secondary partitions in a direc- 

 tion transverse to that of the principal septa, it would be converted into 

 a Heterostegina ; just as a Peneroplis would be converted by the like subdi- 

 vision into an OrUculina ( 464). Moreover, we see in Heterostegina, as 

 in Orbiculina, a great tendency to the opening-out of the spire with the 

 advance of age; so that the apertural margin extends round a large part 

 of the shell,, which thus tends to become discoidal. And it is not a little 



FIG. 338. 



FIG. 339. 



He terostegina. 



Section of Orbitoides Fortisii, paral- 

 lel to the surface; traversing at a. a, the 

 superficial layer, and at 6, b, the median 

 laj'er. 



curious that we have in this series another form, Cydodypeus, which bears 

 exactly the same relation to Heterostegina, that Orbitolites does to Orbi- 

 culina; in being constructed upon the cydical plan from the commence- 

 ment, its chamberlets being arranged in rings around a central chamber 

 (Plate XYI., fig. 1). This remarkable genus, at present only known by 

 specimens dredged up from considerable depths off the coast of Borneo, 

 is the largest of existing Foraminifera; some specimens of its discs in 

 the British Museum having a diameter of 2 inches. Notwithstanding 

 the difference of its plan of growth, it so precisely accords with the Num- 

 muline type in every character which essentially distinguishes the genus, 

 that there cannot be a doubt of the intimacy of their relationship. It 

 will be seen from the examination of that portion of the figure which 

 shows Cydodypeus in vertical section, that the solid layers of shell by 

 which the chambered portion is inclosed are so much thicker, and con- 

 sist of so many more lamellae, in the central portion of the disk, than 



