1899] MICROSCOPICAL JOtlRNAL. 7 



The following is an enumeration of the steps in tlie 

 evolution: — 



1st. A small equilateral triangle is formed at the in- 

 tercostal junction of the polygonal cells (Fig. 2). 



2nd. The angles of the triangle become blunted. 



3rd. The blunted end becomes notched (Fig. 4). 



4th. The notches deepen, and eventually becoming cir- 

 cular, form a perforation at each intercostal angle. At 

 the same time the sides of the triangles form a larger 

 perforation betAveen them. The larger perforations are 

 situated at the bisection of the sides of the hexagons 

 (fig. 9). 



5th. The peripheral perforations situated at the inter- 

 costal angles break into two, and by repeated subdivi- 

 sions form the central finely perforated membrane 

 (Figs. 6 and 7). 



6th. The peripheral perforations, when the central 

 membrane is complete, become more or less of a uni- 

 form size, and then break up into tertiaries (Fig. 8). 



7th. Kepetitions of the sixth process produce a uni- 

 form 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 repeat- 

 ing 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 be- 

 low it precisely similar tt)an Asteromphalus, but having, 

 in place of the well-known caps to the cells, a delicately 

 perforated membrane extending over the whole surface of 

 the valve. The suggestion (merely a suggestion, for I am 



