944 SUMMARY OF CURRENT RESEARCHES RELATING TO 



independent system of very fine circular dots in radiating lines, and 

 some of these are seen at the bottom of the bright areolae when the 

 diatom is examined by transmitted light. 



Coscinodiscus Oculus-Iridis has been fully dealt with supra, p. 941, 

 in Dr. Cox's criticism of MM. Prinz and Van Ermengem's paper. 



Leaving the bolder marked forms, in regard to which the existence 

 of areolae in the valves is so jjlainly shown by the lines of fracture 

 that there has been little or no dispute about it for some years, Dr. 

 Cox takes up the species and varieties which have much finer 

 markings, and with which the difficulty begins. His remarks will be 

 considered of such interest that we transcribe them in full. 



" The most satisfactory method of examination will be found in a 

 progressive study of specimens from each of the more important 

 groups and families, beginning with those having the larger features 

 and passing on to the more delicate. We shall first notice that in 

 the great variation in size which occurs in all species of diatoms we 

 have presented to us examples with a considerable range of diminish- 

 ing areolae also. In different individuals of the same size there is 

 also often found much difference in fineness of areolation. The 

 gigantic forms of Coscinodiscus Oculus-Iridis found in the Maryland 

 deposits become as small as C. radiatus, and the latter is often found 

 in recent marine gatherings side by side with C. subtilis, and of no- 

 greater size. 



We are able, therefore, to follow the diminution of undoubted 

 hexagonal areolae from the greatest of these specimens, where the 

 valves measure 'OIG in. in diameter, till they are scarcely one-eighth 

 as large. Then taking up C. subtilis with its hexagons in the larger 

 valves as clearly marked in outline, we find another diminishing 

 series, in which the sharpest scrutiny still leaves us in doubt when 

 we pass from the hexagonal form to that of round punctee. In this 

 progression we find that the areolae continue to be the weak places in 

 the shell, the fracture following them in the smaller as in the larger 

 examples. Examined by aid of the vertical illuminator, the surfaces 

 of the valve continue to show the characteristic reticulation and ' eye- 

 spots ' as long as we can trace distinct form at all. As the hexagons 

 become smaller we see by transmitted light that they show more 

 colour when the tube is lowered a little, and they are thus brought 

 a little within the focus. In the smallest of these in which we can 

 clearly define the hexagonal outline, the spot becomes quite deeply 

 red. If we next select a valve in which the dots are a little more 

 distant from each other and evidently round (the scheme of marking 

 and the marginal spines being the same as in the larger specimens), 

 we shall find the same conduct with regard to colour when the 

 objective is lowered or raised ; that the fracture indubitably follows 

 the line of the dots, and that under the vertical illuminator the 

 smaller dry specimen is not distinguishable from the larger except 

 in the roundness of the areola. 



Pass now from C. subtilis, as we find it along our own coast in gather- 

 ings shown in Peticolas's slides from Jacksonville or Fernandina, 

 ria., to the Odontodiscus subtilis of Moller's type-plate or his slides of 



