146 MEMBrvANIPORID^. 



It is of course easy to understand how they may have 

 been derived from a common and not very remote ancestor 

 through a process of gradual modification ; but in their 

 actual condition they seem to be sufficiently distinct and 

 stable forms'^. 



M. lineata differs from M. craticula in the larger size 

 and much less regular arrangement of its cells, and in 

 the character of the spines, which are rounded, and not 

 flattened, fewer in number, and much less recumbent 

 than in the other form. The disposition of the spines in 

 M. craticula is very distinctive, and gives a decided indi- 

 viduality to the species. There are also differences in the 

 ovicells. The avicularium of the present species is large 

 and much raised, and projects very prominently at the 

 base of the cell. 



Towards the margin of the colony the cells are sometimes 

 slightly produced below the aperture, so as to assume to a 

 certain degree the form which we have in the normal M. 

 monostachys. Round the lower part of the zooecia there are 

 several rather large oval orifices (intercellular communica- 

 tions), one placed at the top and bottom, and two on each 

 side. On the inner edge of these foramina there are gene- 

 rally a few minute denticles. 



* The truth seems to be that the difference between Prof. Smitt and my- 

 self lies chiefly in the conception we have respectively adopted of a species. 

 By a " form " he intends to denote what I should name in a large propor- 

 tion of cases a species. His "species" is a block of " forms," the various 

 elements of which are supposed to be so related and interconnected as to 

 constitute an evolutional series which is properly regarded as a whole. 

 But to sustain this view an amount of variability is assumed of which I can 

 find no sufficient proof ; I have certainly had no experience of the " innu- 

 merable transition-forms" {"otalika mellanformcr '' ), which, according to 

 Prof. Smitt, bind together M. craticula and M. lineata. That they are not 

 distantly related I have no disposition to deny ; but whatever their histoyy 

 may have been in the past (and the same may probably be said of the other 

 iiYo species which Prof. Smitt refers to his lincaia-stock), they are now dis- 

 tinct and well-established forms, and should be treated as such in our sys- 

 tems. We must have much more evidence of variability than we yet possess 

 to justify us in merging them in one specific group. 



