250 PROF. S. J. HICKSON ON THE [Apr, 5, 



particular set of the tides and currents, and the chemical com- 

 position of the sea-^^'ater, particularly as regards the amount of 

 calcium carbonate it holds in solution, vary very considerably in 

 different reefs and in different parts of the same reef. Such 

 variations must affect the rate of growth of Millepores, and I 

 think it is reasonable to believe the mode of growth also. 



(2) Tlie Size of the Pores. — Dana, Milne-Edwards and Haime, 

 and Quelch have used the size of the pores as a specific character, 

 but, with one exception to be referred to presently, they give 

 no measurements, being contented to use the expressions " very 

 small," " large," " minute," &c. Unless the zoologist has an 

 immense number of specimens from different localities to compare 

 one with another, it is difficult for him to understand what is 

 meant by such expressions : but even the naturalists of the great 

 national collections would be mystified by the case of 31. alcicornis, 

 whose gastropores are accoi-ding to Quelch xevy large, and accord- 

 ing to Milne-Edwards and Haime " tres petits." I have measured 

 a very large number of gastropores, taking for each specimen an 

 average of 6 or 12. 



The greatest average diameter of the gastropores I have found 

 is 0-37 mm., the smallest is 0'13 mm., so that the difference 

 between those pores which might legitimately be called "very 

 large "and those that are " very small " is 0-24 mm. But these 

 " large " pores are very rarely seen ; the great majority of the 

 gastropores are between 0-3 mm. and 0-2 mm. This general 

 result agrees fairly well with the only measurement I have been 

 able to find in the literature of the subject, namely that of 

 M. murrayi by Quelch, which is given as 0*25 mm. 



The question that had next to be considered was whether there 

 is any other feature constantly associated with large pores and 

 with small pores. The large pores are very constantly found in 

 specimens with thick lamellae or branches, while the small pores 

 are found on those of a more slender habit. 



A further investigation of the question yielded an explanation 

 of the variation in the size of the gastropores, which proves that 

 it cannot be of any real service for specific distinction. 



I found that in the gastropores of specimens of slender growth 

 there are only 3 or 4 tabuhe, while in those of more massive 

 growth there may be as many as 9 or 10 tabulae. This suggested 

 that the size of the gastropores depends upon the age of the 

 gastrozoid which lived in it, and, on measuring carefully a number 

 of gastropores from the base, middle branches, and growing-points 

 of a specimen in the Manchester Museum labelled M. compJanata, 

 I found that the average diameter of the gastropores at the base, 

 which we may assume in this case to be the oldest part, was 

 0'185 mm., on a middle branch 0-17 mm., and at the growing- 

 edge, i. e. the youngest part, it is only 0'13 mm. This general 

 result was confirmed by similar series of measurements on other 

 specimens. I also found that the greatest average diameter of 



