190 OEIGINAL ARTICLES. 



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

 thus we seem justified in the belief that whether either 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 conditions 

 under which they have been respectively propagated. 



But to what other type does Peneroplis itself present the closest 

 approximation ? By systematists in general, the intimate relationship 

 which I have shown it to possess to the helical type of Orbiculina has 

 been so slightly regarded, that it has been considered as at least equally 

 related to the Operculina type ; and yet, as I shall presently show, 

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

 features of their structure, as far as any two polythalamous Foramini- 

 fera can be. And the idea of the derivation of Peneroplis from the same 

 stock with Orbiculina seems justified by the fact that the young forms 

 of the two are frequently so alike as not to be distinguishable by exter- 

 nal characters alone, whilst their internal difference consists only in the 

 presence or the absence of the secondary or transverse septa — a character 

 which I have shown reason to regard as variable in this group.* 



Notwithstanding, therefore, the apparently wide divergence of the 

 cyclical Orbitolites, the helical Orbiculina, the fusiform Alveolina, and 

 the simply-chambered Peneroplis 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 com- 

 mon 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 porcel- 

 lanous ; presenting an opaque white hue when seen by reflected light, 

 but a rich brown or amber colour, when seen by light transmitted 

 through thin natural lamellae or artificial sections. This substance is 

 entirely structureless, and possesses no great density or tenacity. More- 

 over, in all the foregoing types, each of the septa intervening between 

 the chambers consists of only a single layer ; and the passages of com- 

 munication 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 sepa- 

 ration, it is here that variations in the mode of communication between 

 the chambers seem to be of least account. It is in this type that we re- 



* My statement on this point is fully confirmed by Messrs. Parker and Rupert 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 sarcode, in- 

 stead of numerous minute, sub-cubical blocks." See Ann. of Nat. Hist., March, 1860, 

 p. 180. 



