86 EDWARD M. NELSON ON DIATOM STRUCTURE. 



7th. Repetitions of the sixth process produce a uniform and 

 delicate perforated membrane over the whole of the valve. 



Beyond the sixth we cannot with certainty go, for we have no 

 evidence concerning the seventh or final stage. The sixth stage may 

 be the last, or it may go on repeating itself, after the manner of 

 the fifth, until there is formed, not merely a cap to each polygonal 

 cell, but a complete membrane covering the whole surface of the 

 valve ; and further, may not this be the adult form of the valve ? 



I have frequently seen such valves : they consist of a primary 

 honeycomb structure, with an eye-spot layer below it precisely 

 similar to an Astero7nphahcs, but having, in place of the well- 

 known caps to the cells, a delicately perforated membrane ex- 

 tending over the whole surface of the valve. The suggestion 

 (merely a suggestion, for I am 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 mem- 

 brane is the adult form of the diatom known as CoscinocUscus 

 Asteromjjhalus, and that this is formed by the continual repetition 

 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 conclusion, let me say that the diatoms were 

 from the Nottingham 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 Mr. Ingpen's slide, and another old slide mounted by 

 Moller : probably careful search would reveal the structure in 

 almost any slide containing Nottingham Coscinodisci. 



Let us in conclusion hark back for a moment to our first step. 

 It would be very interesting to learn what was really the te7'mi7ius 

 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 ve^'sd 1 As there is nothing particu- 

 larly 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 larger number 

 of primaries and consequently narrower intervening silex ; others 



