STRUCTURE OF NUMMULITES. 



513 



the other parts ; and in Nummulites whose surfaces have been much 

 exposed to attrition, it commonly happens that the pillars of the 

 superficial layer, being harder than the ordinary shell-substance, 

 and being consequently less worn down, are left as 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 preceding whorl ; this passage is sometimes a single 

 large broad Aperture, but is more commonly formed by the more or 

 less complete coalescence of several separate perforations, as is seen 

 in Fig. 256, 6. There is 



also, as in Operculina, a p IG ogo. 



variable number of isolated 

 pores in most of the 

 septa, forming a secondary 

 means of communication 

 between the Chambers. — 

 The Canal-system of Num- 

 muUna seems to be dis- 

 tributed upon essentially 

 the same plan as in 

 Operculina; its passages, 

 however, are usually more 

 or less obscured by fossil- 

 izing material. A careful 

 examination will generally 

 disclose traces of them in 

 the middle of the parti- 

 tions that divide the 

 Chambers (Fig. 259, 6, b), 

 while from these may be 

 seen to proceed the lateral 

 branches (c, c), which 

 after burrowing (so 



speak) in the walls of the between the chambers, 

 chambers, enter them by 



large orifices (d). The Interseptal Canals, and their communication 

 with the inosculating system of passages excavated in the Marginal 

 Cord, are extremely well seen in the Internal Cast represented in 

 Fig. 260. 



394. A very interesting modification of the Nummuline type is 

 presented in the genus Heterostegina (Fig. 261), which bears a very 

 strong resemblance to Orbiculina in its Plan of growth, whilst in 

 every other respect it is essentially different. If the principal 

 chambers of an Operculina were divided into Chamberlets by 

 secondary partitions in a direction transverse to that of the principal 

 septa, it would be converted into a Heterostegina; just as a Penc- 



L L 



Cast of the Interior of two of the 

 Chambers, a, a, of Nummulina striata, with 

 V' the network of Canals, b, b, in the Marginal 

 "° Cord, communicating with canals passing 



