12 DE. CAEPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



ceeded so far as to produce a number of separate branching apertures, and nothing 

 is wanting but the removal of the line of shell which passes down the middle of the 

 septum, to unite these into the most characteristic form of the single ramifying orifice. 

 This individual, like the one represented in fig. 6, was already beginning to assume the 

 Spirolina form, the rounded shape of the mouth showing that the spire has detached 

 itself completely from the previously-formed convolutions: in figs. 4 and 11 a are shown 

 the mouths of more advanced examples of the same type, which present such a com- 

 bination of the large dendritic aperture of Dendritina with the isolated pores of Pene- 

 roplis, as to complete (in my opinion) the proof that no valid distinction can be drawn 

 between these two types, either from the number, the isolation, the position, or the 

 shape of the apertures in the septa. This conclusion will probably become less sur- 

 prising than it may at first sight appear to such as are accustomed to the study of the 

 more constant characters presented by the shells of Mollusca, when it is borne in mind 

 that the apertures seem to have no other function than to give passage to threads of 

 sarcode ; and that it is the general character of the Ehizopod type, for such extensions 

 of the substance of the body to be entirely destitute of constancy either as to position 

 or number. 



138. If the foregoing conclusion be admitted, it follows that not merely must the 

 genera Dendritina and Spirolina be relinquished, but that both these forms must be 

 regarded as mere varieties of Peneroplis planatus. From the circumstance that the Den- 

 dritina-tyTpe is found only in tropical seas, and that it attains a far larger size than the 

 ordinary Peneroplis-type, whilst the latter seems to be almost starved out (as it were) 

 in the Mediterranean, presenting itself there under its humblest form, and scarcely 

 extending itself at all into northern seas, I am disposed to think that temperature has 

 a considerable influence in producing these varieties. I should be far, however, from 

 attributing the entire result to this agency alone, since we find Peneroplis presenting 

 itself abundantly in tropical seas, along with Dendritina. But as it is in the Mediter- 

 ranean forms that we meet with the extreme of flattening and with the most uniform 

 singleness of the row of pores, whilst it is among the tropical forms that we find Pene- 

 roplis not merely attaining its greatest dimensions, but also presenting the closest 

 approximation to Dendritina in the turgidity of its spire and in the arrangement of its 

 septal apertures, it seems obvious that the development of this type of structure is 

 favoured by a constantly high temperature ; and further, that Dendritina may be 

 regarded as the highest form, towards which Peneroplis tends in proportion as it is 

 subjected to that influence. As I have not had the opportunity of examining the 

 fossil forms of this type, I am as yet unable to say how far the foregoing conclusions 

 will be borne out by the phenomena which they present. 



Genus OPEECULINA. 



139. The collection of Mr. CUMING contains a large number of Nautiloid Forami- 

 nifera, differing considerably from one another not only in size and proportions, but also 



