194 ORIGINAL ARTICLES. 



mental skeleton, and by the extension of this into radiating prolongations. 

 But the number, forms, and proportions of these prolongations are sub- 

 ject 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 sup- 

 plemental skeleton must be regarded as a feature of minor importance. 

 On the other hand, the development of radiating out-growths is an oc- 

 currence 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 Ro- 

 talia, with which its affinity in every other particular is most inti- 

 mate. 



If, again, we inquire into the import of that remarkable development 

 of the canal-system, which seems to be the distinctive feature of Polysto- 

 mella (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 pre- 

 sents the most symmetrical and extensive distribution of the canal- sys- 

 tem 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 Ampkistegina Cumingii 

 and the comparatively diminutive A. gibbosa; as also (4th series) between 

 the two forms of Tinoporus, where its presence or absence is obviously as- 

 sociated 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 adja- 

 cent 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 cham- 

 bers 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- 



