VI PREFACE. 



(1) the extreme latitude of the rm>ge of variation in this group, which breaks down in ahnost 

 every instance the boundaries wliieh it has Ijcen attempted to erect between species ; (2) the 

 necessity of a hke abolition of the divisions between many reputed genera which have been 

 erected on an equally insecure basis ; (3) the completely unnatural character of any system 

 which makes a fundamental division between the INIonothalamous and the Polythalamous 

 types, and which adopts Plan of Growth (that is, the geometrical arrangement of aggrega- 

 tions of successive segments) as the basis of the subdivision of the PoJgthalamia into orders; and 

 (4) the fundamental importance, in the determination of the true affinities of the several 

 generic types, of all that relates to the physiological condition of the animal, especially the 

 texture of the shell, and the peculiarities of conformation which characterise its individual 

 segments. 



Not only, moreover, did there prove to be this complete harmony in our general results, 

 but there was also a singular unity in the aggregate of the work we had respectively accom- 

 plished, each portion being, so to speak, the complement of the other ; so that, on comparing 

 notes, we found that we had between us pretty thoroughly investigated the entire group. 

 Hence I was led to propose to the Council of the Ray Society an enlargement of my original 

 plan, so as to include the results of my friends' labours, and to render the whole an expression 

 of our joint views. This I did in the expectation that wo might associate ourselves together 

 in such a manner that whilst the general plan and a part of the details of its working out 

 would rest with me, a large share in the execution would be taken by my coadjutors. We 

 soon found, however, that it would be more conducive both to unity of design and to com- 

 pleteness of effect for the whole to be wrought out by myself; and it has been by the 

 necessity which thence arose for my personal study of many types with which I was previously 

 but little or not at all acquainted, that the delay in the production of the work has for the 

 most part been occasioned. The materials for this study have been most liberally supplied to 

 me by Messrs. Parker and Rupert Jones ; and as to many types which they had previously 

 made the object of special researches (such as the Milioline, Nodosarine, Texfidarine, and 

 Botali/ie grou\)s'), I have found that I had nothing to do but to accept their well-considered and 

 satisfactory conclusions. In certain other cases, especially in regard to the genus Bacii/hpora 

 and to that collection of forms which they had described under the generic designation 

 Orhitolina (here referred to the genera Tinoporus and Pafellina), my own investigation of the 

 materials which they have placed in my hands has led me to results in some respects different 

 from those which they had published ; but as they have seen reason to accept my modifica- 

 tions, the account of those types here given may be regarded as not less theirs than mine. In 

 regard to the genus Nummidina, the most important cf all Poraminifera in a geological point of 

 view, we have found ourselves in complete accordance as to the impossibility of drawing definite 

 lines of demarcation between its reputed species; my researches on the varietal forms of the 

 closely related genus Ojjercidina having led me to conclusions as to the variability of all the 

 differential characters on which reliance had been placed, precisely Qorresponding wdth those at 



