574 DE. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



transitional conditions of aperture, are so obviously related to both the foregoing, that 

 no reasonable doubt can exist as to their derivation from them. Now the geographical 

 distribution of the two fundamental types is so far different, that where one prevails the 

 other is either absent altogether, or presents itself under a modified form ; and thus we 

 seem justified in the belief that, whether either of them has been derived from the other, 

 or both have been derived from some intermediate form (such as that which seems 

 common alike to the young of both), the modifications which have given rise to the 

 marked differences they now exhibit are mainly due to diversities in the external con- 

 ditions under which they have been respectively propagated. 



244. But to what other type does Pencroplis itself present the closest approximation 1 

 By systematists in general the intimate relationship which I have shown it to possess to 

 the helical type of Orbicullna (^f 130) has been so slightly regarded, that it has been 

 considered as at least equally related to the Operculine type ; and yet, as I shall pre- 

 sently show, these two types are removed from each other in all the most essential 

 features of their structure, as far as any two polythalamous Foraminifera can be. And 

 the idea of the derivation of Peneroplis from the same stock with OrMculina, seems 

 justified by the fact that the young forms of the two are frequently so alike as not to 

 be distinguishable by external characters alone, whilst their internal difference consists 

 only in the presence or absence of the secondary or transverse septa, a character which 

 I have shown reason to regard as variable in this group* (^[ 130). 



245. Notwithstanding, therefore, the apparently wide divergence of the cyclical Orbi- 

 tolitcs, the helical Orbiculina, the fusiform Alveolina, and the simply-chambered Pene- 

 roplis and Dendritina, these several types must be regarded as most intimately related 

 to one another; and that relationship seems to me much more likely to have arisen 

 from a common ancestral descent, than from the original creation of independent types 

 capable of graduating into each other so continuously as almost to assume each other's 

 characters. It is very important to remark that they all possess that peculiar texture of 

 shell, which is designated by Professor WILLIAMSON as porcellanous ; presenting an opake 

 white hue when seen by reflected light, but a rich brown or amber colour when seen by 

 light transmitted through thin natural lamella? or artificial sections. This substance is 

 entirely structureless, and possesses no great density or tenacity. Moreover in all the 

 foregoing types, each of the septa intervening between the chambers consists of only 

 a single layer ; the passages of communication between them are for the most part so 

 large and free, that the segments of the sarcode-body are but very imperfectly isolated 

 from each other ; and, as might be anticipated from this incompleteness of separation, it 

 is here that variations in the mode of communication between the chambers seem to be of 



* My statement on this point is fully confirmed by Messrs. PAEKEB and RTTPEET JONES ; who state that 

 " not unfrequently, feebly- developed peneropliform varieties, as well as good-sized adunciform specimens, 

 occur, in which the long narrow chambers are at times simple and undivided, being occupied by transversely 

 elongate lobes of Barcode, instead of numerous minute subcubical blocks." See Ann. of Nat. Hist., March 

 1860, p. 180. 



