CABPENTEE ON FOBAMINIFEBA. 189 



linear axis; and wc find this axis in Orliculina, sometimes equalling in 

 length the diameter of the spire, so that this organism at an early stage of 

 its growth may be nearly spheroidal. Now among the various types of 

 fossil Alveolina, there are some whose shape, instead of being fusi-form, 

 like that of the recent type I have described, is almost identical with 

 that of a spheroidal Orliculina ; and the general structure of two such 

 organisms will be so nearly identical, that I cannot see any difficulty 

 in referring them to a common original. And when we examine a 

 series of such fossil types, we see that the}' present a wider and wider 

 divarication from the Orliculina type in this one particular alone, that 

 whilst the later growth of Orliculina tends to liken it to Orbitolites, that 

 of Alveolina tends to the continual elongation of its vertical axis — a dif- 

 ference which all analogy would indicate to be one of far too small 

 account in this group to be justly taken as a ground of original dis- 

 tinction. 



In the assemblage of forms which I have thought myself justified in 

 re-assembling under the designation Peneroplis (3rd series), we encounter 

 other remarkable series of variations, the principal of which have given 

 occasion to the formation of the two additional genera Dendritina and Spi- 

 rolina. With an exceedingly close conformity in the texture and in the 

 superficial markings of their shells, as well as in their general plan of 

 growth, we observe a marked diversity in the form and proportions of 

 the spire, especially in the later stages of its growth, and a still greater 

 divergence in regard to the form and disposition of the septal apertures. 

 For in the type to which M. D'Orbigny restricts the generic designa- 

 tion Peneroplis, we usually find the spire rapidly widening and becom- 

 ing proportionally compressed in each succeeding convolution ; whilst 

 in that which he distinguished as Dendritina, the spire widens but 

 slowly whilst increasing rapidly in turgidity. Further, in the one 

 type, as in the other, the later extension is often in a straight line, 

 instead of continuing to follow the spiral course ; and on this variation 

 alone, which (as will presently appear) is of no account whatever among 

 Foraminifera, has been erected the genus Spirolina. Now, in the typical 

 Peneroplis, the septal plane presents a single linear series of minute 

 rounded pores, whilst in the typical Dendritina we find in their place 

 a single large orifice with radiating extensions, the difference between 

 these two modes of communication being as great as we find between al- 

 most any two types of Foraminifera whatever. Yet I believe that no one 

 who will go through the details of the evidence" I have collected from 

 the study of transitional forms, will have any doubt that Peneroplis and 

 Dendritina may have had a common progenitor, and that the peculiarity 

 in the mode of septal communication that characterises each is intimately 

 related to the compressed or turgid form of the spire in each case ; whilst 

 the different forms of Spirolina t}-pe, among which we find the most 

 remarkable transitional conditions of aperture, are so obviously related 

 to one or other of the foregoing, that no reasonable doubt can exist of their 

 derivation from these. Now, the geographical distribution of the two 

 fundamental types is so far different, that where one prevails, the other 



