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



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 cmnot 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 tabulae, while in those of more massive growth there 

 may be as many as 9 or 10 tabulee. This suggested that the size of 

 the gastropore 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 specimem in the Manchester 

 Museum labelled M. complanata, 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 

 gastropores which I have given above was obtained from the base of a 

 massive specimen, while the smallest was obtained from a growing-edge 

 of a slender specimen. 



Moreover, it occurred to me that if the size of the gastropores is de- 

 pendent upon their age or the rate at which the gastrozooids have 

 grown, there ought to be, in some cases at any rate, a difference 

 between the average size of the gastropores on one side of a branch or 

 plate and that on the other ; those on the face most favourable as re- 

 gards food supply in the living state should be larger than those on 

 the other. Measurements confirmed my point, and I found a difference 

 in two out of three specimens between the gastropores on one side 

 and those on the other as great as 0"03 mm. 



(3) The Degree of Isolation of the Cycles. — Moseley noticed that in one 

 specimen of Millepore taken at Zamboanga the cycles were much more 

 distinct than in other specimens, and suggested that this feature 

 might be of specific value. After very careful consideration I am con- 

 vinced that it cannot be. In many large specimens it will be seen that 

 the cycles are much more distinct in one part than another. Some- 

 times the cycles are so crowded as to be indistinct at the edge, and 

 perfectly clear on the face or at the base. The evidence points to the 



