mental skeleton, and by the extension of this into radiating prolongations. But the number, forms, and proportions of these prolongations are subject to very considerable variations; so that, whilst they are sometimes so greatly multiplied and prolonged as to constitute the principal feature of the organism, they are so little developed in other instances, that the contour of the disk is scarcely interrupted by them. Further, I have shown that the development of this supplemental skeleton is, in a great degree, independent of that of the spire; hence, if this last be the essential component of the organism (as all analogy indicates), the supplemental skeleton must be regarded as a feature of minor importance. On the other hand, the development of radiating out-growths is an occurrence not unfrequent among other helicine Foraminifera, even in species whose typical form is altogether destitute of them (as Professor Williamson has pointed out in Polystomella crispa); and such forms differ much less widely, as regards this character, from the simpler forms of Calcarina, than these last do from the very complex forms with which they are connected by a continuously-gradational series. Hence, I cannot regard the remarkable development of the supplemental skeleton in Calcarina as affording any disproof of its genetic relationship to Rotalia, with which its affinity in every other particular is most intimate.
If, again, we inquire into the import of that remarkable development of the canal-system, which seems to be the distinctive feature of Polystomella (4th series), we find that if we base our judgment upon a sufficiently wide foundation of facts, its non-essential character becomes apparent. For although the large P. craticulata of the tropical and Australian seas presents the most symmetrical and extensive distribution of the canal-system that I have anywhere met with, the little P. crispa of our own seas exhibits but feeble traces of it; yet of the intimacy of their relationship no doubt can be fairly entertained. I have shown (3rd series) that a parallel difference exists between the gigantic Amphistegina Cumingii and the comparatively diminutive A. gibbosa; as also (4th series) between the two forms of Tinoporus, where its presence or absence is obviously associated with the presence or absence of the radiating prolongations, and of the supplemental skeletons from which these proceed.
In considering the import of the canal-system as a character for the systematist, the mode of its formation must not be left out of view. I have shown that the passages which altogether go to make up this system are not true vessels, but are mere sinuses, left in some cases by the incomplete adhesion of the two contiguous walls which separate adjacent chambers, and in other cases apparently formed by the incomplete calcification of the sarcode which forms the basis of the solid skeleton; certain portions of that substance remaining in their original condition, so as to maintain a communication between the contents of the chambers and the parts of the shelly casing most removed from them, just as the fissures or pores which communicate between the chambers, and between the last chamber and the exterior, are mere unconsolidated portions of the septa, occupied in the living state by commissural por-
