140 



SYDNEY J. HICKSON, M.A., D.SC, P.R.S., F.Z.S. 



and the other is not, or that one lamella or branch is covered with wart- 

 like processes and the others are smooth. 



(6) The Relative Number of Dactylopores and Gastropores. — Finding 

 that all other characters derived from the skeleton are unsatisfactory 

 for determining and distinguishing species, I thought it possible that a 

 good character might be found by calculating the average number of 

 dactylopores to each gastropore in a number of species. 



In many specimens the cycles are so close one to another that it is 

 often difficult to determine to which cycle a particular dactylopore 

 belongs. In order, therefore, not to be misled, I used only those cycles 

 which were clearly defined from their neighbours. 



In the following table I have put together the results of my calcula- 

 tions on this point : — 



Accepted specific names 

 of specimens. 



(The name of donor and 

 locality in parentheses.) 



Number of 

 cycles 

 counted. 



Average No. of 

 dactylopores 

 in each cycle. 



Highest 

 number. 



Lowest 

 number. 



I. M. murrayi. 



(Hadclon, Torres Str. ) 



II. M. alcicomis. 



(Brit. Mus., W. Indies/ 



III. M. alcicomis. 

 (Shipley, Bermudas.) 



IV. M. alcicomis. 

 (Lister, Tonga.) 



V. M. plicata. 

 (Hiekson, Celebes. ) 



VI. M. complanata. 

 (Man. Mus.,W. Indies.; 



VII. M. alcicomis. 

 (Man, Mus., W. Indies.) 



VIII. M. alcicomis. 

 (Agassiz, Bahamas) 



6 



8 



6 



12 

 12 



12 



7 

 100 



7 

 13 



5-15 



6-45 



5-6 



6-7 

 5-08 



7-08 



6-28 



5-82 



6-14 

 5-5 



8 



8 



/ 



8 

 8 



9 



7 



7 



7 

 9 



5 



5 



5 



6 

 3 



6 



5 

 4 



5 

 4 



It will be seen from these figures that there is not much variation in 

 the average proportion of dactylopores to gastropores in the different 

 forms examined. The largest number of cycles I was able to count on 

 one colony gave an average of a trifle under 6. It is noteworthy that 

 this is the exact mean of the highest and lowest averages obtained from 

 smaller specimens on which only a few cycles could be counted. 



The extreme averages 5 "08 and 7 "08 (IV. & V.) do not show so great 



