96 Rev. T. Hincks's Critical Notes 



pora nitida, in which the extremities of the ribs are closely 

 and permanently united, so as to form a distinct median line. 

 This is an essential character of the only genus Membrani- 

 porella which we know, and it is an essential character of the 

 Cribriline family. Forms in which it is wanting must be 

 placed elsewhere. 



Dr. Jullien regards the MenibraniporellcE as the highest of 

 the Membraniporidce ; to me they are the lowest of the Cri- 

 brilinidce. Let me say at once, however, that I am in perfect 

 agreement with him when he urges that the true Membrani- 

 jjorce, Membraniporella, and the Cribrilinidce are forms which 

 11 s'enchainent et pourraient a la rigueur ne former q'une seule 

 famille." No doubt they are terms in an evolutional series, 

 connected by many transitional links, and on merely genea- 

 logical grounds might well be gathered into a single group. 

 But the question will arise, Why should we stop here? for 

 we shall probably find that the group is not an isolated thing, 

 but touches other groups at many points, and that the family 

 relationship is wide and far-reaching. If we are to have any 

 system at all embracing a number of limited groups the latter 

 must represent the more marked stages in the evolutional 

 process, the new structural departures, as it were, and the 

 boundaries traced around these groups must be treated rather 

 as imaginary lines, drawn for the sake of convenience, than 

 as actual and abiding partition- walls *. For always and in 

 all directions our " distinctive characters " will be gradually 

 changing their aspect and significance, according to the method 

 of nature. Only in this way can we make our classifications 

 correspond with the actual plan of organic life. The Cribri- 

 line family, in my judgment, has been rightly constituted to 

 represent an important morphological advance in the Mem- 

 braniporine tribe. 



Now if we examine this tribe, we find in the first place a 

 series of forms (genus Membranipora) in which the zooecial 

 aperture is wholly closed in by the primitive membranous 

 covering, and there is no trace of a calcareous front-wall ; in 

 some cases the margin of the aperture bears a number of 

 spines or spinules, which may possibly have to some extent 

 a protective function, in others the spines are more massive 

 and bend in over the aperture, so as to form a rude kind of 

 roofing. In some species they are altogether absent. The 



* " In all our classifications of a truly natural group, where the diffe- 

 rent species will be arranged into more or less complete series, we must 

 be prepared for seeiug the limits between the divisions fading away, espe- 

 cially when the developmental changes are known." (Smitfc, ' Floridan 

 Bryozoa,' part 2, p. 41.) 



