242 Transactions of the [Sess. 



Strasburger, Abbe, Weiss, Prinz, van Ermengem, Errera, Deby, 

 Grunow, Stephenson, Slack, Morehouse, Miiller, Wells, Schumann, 

 Pfitzer, Hallier, Borscow, Walker- Arnott, Dippel, Max Schultze, 

 Burgess, Kitton, Cox, and van Heurck have, among others, recently 

 expressed opinions on this most difficult problem, but of these the 

 conclusions arrived at by Cox and van Heurck need alone be selected, 

 in contrast to those of Flogel above quoted, as indicative of the 

 great difficulties that are encountered in the solution of this com- 

 plex question. 



Dr J. D. Cox^ infers generally — (1) That the Diatom shell isusi;- 

 ally formed of two lamina?, one or both of which may be areolated, and 

 may be strengthened by ribs which have been described both as 

 costa? and as canaliculi. (2) That the normal form of the areola^ 

 is circular, but that these, if crowded, become hexagonal or sub- 

 hexagonal. (3) That the areolae are pits or depressions in the 

 inner surface of one of the lamina?, so that when two lamina are 

 applied together, the exterior surfaces of the shell are approxi- 

 mately smooth, and the cavities are within. (4) That the apparent 

 thickening on the exterior of the lines bounding the areola? in some 

 species (Eupodiscus argus) is not in contravention of, but is in 

 addition to, the formation above described. (5) That however fine 

 the dotted markings of Diatom valves may be, the evidence from 

 the colour of the spaces between the dots, and of the dots them- 

 selves, supports the conclusion that they follow the analogy of the 

 coarser fornas, in which both fracture and colour are found to prove 

 that the dots are areola? and the weaker places in the shell. 



Dr van Heurck ^ has, on the other hand, stated his general con- 

 clusions as follows : — 



(I.) The valves in the Cryptoraphidiea? generally consist of two 

 layers — ■ 



(a) The lower single lamina, with more or less delicate 

 punctations on its inner face. 

 (It could not be determined whether the punctations 

 completely traversed the lower layer, or only pro- 

 ceeded to a certain depth.) 

 (/3) The upper lamina varies — 



(i) It forms alveoli, usually closed from above, in 

 Triceratium favus and Coscinodiscus oculus- 

 iridis. 

 (ii) The alveoli are open above in Eupodiscus argus. 

 (iii) In some cases the upper layer may entirely 

 disappear (Triceratium intricatum), the various 

 stages in this process being traceable in dif- 

 ferent species. 



^ 'Jour. Roy. Micr. Soc.,' Dec. 1884. 



- ' Syuop. d. Diat. d. Belgique,' pp. 35-37. 



