834 



MICKOSCOPIC FORMS OF ANIMAL LIFE 



FiG."634, Internal cast of two of the cham- 

 bers of Nummulites striata, with the 

 network of canals, &, in the marginal 

 cord communicating with canals passing 

 between the chambers. 



prominences, the presence of which has often been accounted (but 

 erroneously) as a specific character. The successive chambers of the 

 same whorl communicate with each other by a passage left between 



the inner edge of the partition 

 that separates them and the 

 ' marginal cord ' of the pre- 

 ceding whorl ; this passage is 

 sometimes a single large broad 

 aperture, but is more com- 

 monly formed by the more or 

 less complete coalescence of 

 several separate perforations, 

 as is seen in fig. 631, b. There 

 is also, as in Operculina, a 

 variable number of isolated 

 pores in most of the septa, 

 forming a secondary means of 

 communication between the 

 chambers. The canal system 

 of Nummulites seems to be ar- 

 ranged upon essentially the 

 same plan as that of Oper- 

 culina ; its passages, however, 



are usually more or less obscured by fossilising material. A careful 

 examination will generally disclose traces of them in the middle of 

 the partitions that divide the chambers (fig. 633. b, b), while from 

 these may be seen to proceed the lateral branches (c, c), which, after 

 burrowing (so to speak) in the walls of the chambers, enter them 

 by large orifices (d). These ' interseptal ' canals, and their communi- 

 cation with the inosculating system of passages excavated in the 



marginal cord, are extremely 

 well seen in the * internal cast ' 

 represented in fig. 634. 



A very interesting modifi- 

 cation of the Nummuline type 

 is presented in the genus 

 Heterostegina (fig. 635), which 

 bears a very strong resemblance 

 to Orbiculina in its plan of 

 growth, whilst in every other 

 respect it is essentially dif- 

 ferent. If the principal cham- 

 bers of an Operculina 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 subdivision into 

 an Orbiculina. Moreover, we see in Heterostegina, as in Orbiculina, 

 a great tendency to the opening out of the spipe with the advance of 



