540 PLANKTON OF THJi SYDNEY WATEU- SUPPLY, 



The following dimensions indicate in each case the range 

 of five or six specimens: — 



In the case of the 3rd Ant. horn, at any rate, it was quite 

 evident that the horns grow out from the body : every size 

 could be observed, from almost nothing up to 50 /x. 



Genus Peridinium Ehr. 



The filter-samples yielded abundance of I'eridinui of all 

 shapes and sizes, but although several types could be easily 

 distinguished, there were many indications to show that they 

 were all the various stages of growth and development of a 

 single species. The slanting excavation at the back of the 

 hypovalve is common to forms of every size, as is also the 

 extremely broad vertical furrow with which it is connected, 

 and the tooth on either side. The two smaller teeth close 

 together, at the summit of the epivalve in the larger forms, 

 are the direct outgrowths of the short, square, produced apex 

 in the smaller ones. Again, a perfect series of sizes could 

 easily be obtained from 20 ju, to 80 /a in length. The following 

 figures, which are merely the dimensions of those sketched for 

 reproduction, sufficiently indicate this: — 20x16, 24x18, 30 



X 21, 30 X 27, 32 x 28, 34 x 24, 35 x 27, 38 x 35, 42 x 29, 44 x 

 38, 44 X 40, 50 x 46, all smooth membranes ; 60 x 53, 63 x 57, 

 63x60, 63x63, 70x67,76x70, 76x76, all granular; while 

 the fact that the ridged forms— 44 x 42, 46 x 42, 46 x 44, 52 



x52, 54x54, 54x59, come in just at that point when the 

 granulation is about to form on the hitherto smooth mem- 

 brane, shows quite plainly that their incrassate ridging is 

 due to some sudden influence at that critical period — stagna- 



