S THE AMERICAN MONTHLY [Jan 



quite unable to prove it) I now throw out is that a valve 

 consisting of polygonal primaries entirely covered with 

 a uniform and delicately perforated membrane is the 

 adult form of the diatom known as Coscinodiscus aster- 

 omphalus, and that this is formed by the continual repe- 

 tition of the seventh stage. 



All who have conscientiously worked at these images 

 will, I believe, admit that all the stages up to the sixth 

 are proved. With regard to the last, it may be left sub 

 judice until further evidence is forthcoming. In conclu- 

 sion, let me say that the diatoms were from the Notting- 

 ham deposit, and the slide was kindly given to me by 

 Mr. Ingpen. All the stages up to the fifth inclusive 

 have been observed in many different mounts, but the 

 sixth has only been seen in that slide, and another old 

 slide mounted by MoUer : probably careful search would 

 reveal the structure in almost any slide containing Not- 

 tingham Coscinodisci. 



Let us in conclusion turn back for a moment to our 

 first step. It would be very interesting to learn what 

 was really the terminus ad quern of this species. Is it the 

 primary polygonal structure, or the eye-spot layer ; or is 

 the eye-spot layer formed out of the polygonal structure, 

 or vice versa ? As there is nothing particularly minute 

 about these structures, it ought not to be difficult to 

 find an answer to these questions. 



In these Nottingham slides there are examples of a 

 diatom which possesses large circular primaries with 

 broad intervening silex. Very similar valves are also 

 seen having a large number of primaries and consequently 

 narrower intervening silex ; others have a still larger 

 number, and so on until we find the circular primaries 

 compressed into hexagons. It is important to note that 

 this compression into hexagons commences at the peri- 

 phery of the valve, the last primaries to retain the circu- 

 lar form being situated at the centre of the valve. This 



