306 MR, W. D. LANG ON GROWTH-STAGES [Mar. 2! 
further stage (text-fig. 46, Gt), in which the cost are more wavy, 
occurs before the end of the Trochoid shape-stage. At this point 
the first rejuvenescence occurs, and on beginning growth again in 
the Cylindrical shape-stage the corallum is in stage Ba (text- 
fig. 46, F°), but rapidly passes to stage G (text-fig. 46, G*). After 
the second rejuvenescence the wavy G stage seems to have become 
permanently established, to the exclusion of stage F. 
VIII. Summary. 
Perhaps too much stress has been laid upon what, after all, are 
only a few observations ; but the points introduced are of are 
interest that it is hoped that they will be borne in mind and 
tested when work is done on Corals, and not lost sight of in the 
desire to describe new forms or to prove that too many already 
have been described. Summarily the points touched on are 
these :— Hereditary growth-stages exhibited in Rejuvenescence as 
evidence that the latter is a form of Fission, and subject to the 
same laws as other forms of Fission, such as s Branching ; ; views on 
the formation of Dissepiments and Tabule and Bernard’s ideas 
on these in connection with Fission; their equal application to 
the colony as to the individual and a suggested relation with 
Histolysis ; finally comparison with other widely different groups 
of branching organisms— Plants and Polyzoa, that laws of 
branching in one group may be compared with those in another 
and any law common to all may be determined. 
Nore :—Owing to the present inaccessibility of the collection 
of British Chalk fossils formed by Dr. A. Rowe, it has not been 
possible to test as fully as possible the stratigraphical value 
of these observations; but thanks to the kindness of Messrs. 
G. E. Dibley, C. P. Chatwin, and T. H. Withers, a certain amount 
of zonally-collected material has been available for comparison 
with the specimens here described. This, together with those of 
the British Museum specimens whose zone is known, has made it 
possible to draw up the following scheme of distribution in time. 
It will be seen that the order of forms arranged according to this 
distribution agrees on the whole with that deduced above from 
their structure. 
P. serpentina, and forms resembling it: zone of Holaster planws— 
Micraster coranguinum. 
P. centralis, and forms resembling it: zone of Holaster planus — 
Micraster coranguinun. 
P. fittoni, ; 
P. granulata, and forms resembling them; zone of JV craster 
P. gravesi, coranguinum. 
P. mantelli. 
e Hes iL Zone of Belemnitella mucronata. 
. cylindrica. | 
[8rd April, 1909. 
