EHIZOPODA. 17 



In addition to ttie terminal orifice, which we 

 have hitherto regarded as the sole growing point of 

 the shell, many Foraminifera have the external 

 surface of the latter perforated with numerous 

 minute apertures, through which thread-hke 

 extensions of the sarcode body can be protruded 

 {fig. 3, c); and it is not improbable that, by 

 the coalescence of several of these, a layer is 

 formed which may serve for the deposition of cal- 

 careous matter in the form of spines or other 

 peculiarities of surface configuration. It would 

 seem, however, that in Faujasina, Operculina, 

 and certain other Polythalamia, these foramina 

 are not to be regarded as simple apertures in the 

 w^alls of the chambers, but rather as the orifices of 

 a peculiar system of ' interseptal ' canals, which 

 after ramifying between the walls of contiguous 

 chambers proceed directly to the innermost portion 

 of the shell, serving to bring those parts of the 

 sarcode body which are contained in the latter 

 into immediate communication with the exterior. 

 In Num/mulites and other fossil forms these canals 

 have been observed to increase both in number 

 and complexity of arrangement ; for here, in addi- 

 tion to the regulai' series of chambers, there exists 

 an 'interstitial skeleton' for the nutriment of 

 which this increased development of the * canal 

 system,' would appear to be required. Hence 

 it has been said that these Polythalamia present 

 us with the highest and most fully developed type 

 of Foraminiferous structure. 



A mode of growth distinct from any of the 

 preceding has been observed by Dr. Carpenter 

 to take place in another group of Foraminifera, 

 of which Orbitolites may be regarded as the type. 



C 



