1884.] 



MICKOSCOPICAL JOURNAL. 



86 



insure contact ^vith the slide. Place 

 the -object in the balsam, taking care 

 to see that it is completely covered ; 

 wai^m the cover and place it in po- 

 sition, in doing so holding it in the 

 forceps parallel with the surface of 

 the slide, so as to expel the air all 

 around ; weight down with a Minnie 

 bultet, and apply heat as may be nec- 

 essary to harden the balsam. 



The result of this manipulation you 

 see in the slide sent you by this mail, 

 it having been mounted, cleaned, 

 labeled^ and ringed, in other words 

 finished inside of three hours. What 

 I consider quite a feature is this, that 

 there ^vould seem to be no possibility 

 of varnish running in, the channel in 

 the top of the ring receiving the ex- 

 cess of balsam when pressed out by 

 the cover, and thus forming a barrier 

 to the influx of the varnish used in 

 ringing. For flattening the rings I 

 use. two plates of brass*, 3^ inches 

 square by ^-inch thick. Place the 

 rings, six or more at a time, between 

 the plates ; press in an old notary 

 lever stamp. This method of mount- 

 ing seems to have the follow^ing de- 

 sirable features, viz : no previous 

 preparation and drying of cells, ra- 

 pidity and neatness of finish, and no 

 running in of varnish. 



[The mount which accompanies 

 the above communication is certainly 

 a very neat one. We confess to a 

 little surprise that, the balsam does 

 not run out beneath the loose ring, 

 for w^e have a dim recollection of 

 having tried the method once, but 

 our rings were not flattened. — Ed.] 

 o 



Structure of tlie Diatom-shell. — 

 III. 



BY JACOB D. COX, LL. D., F. R. M. S., 

 PRES. AM. SOC. OF MICR. 



The diatoms of which we have ex- 

 amined the forms have thus far been 

 of the bolder marked kinds, in regard 

 to which the existence of areolae in 

 the valves is so plainly shown by the 

 lines of fracture that there has been 

 little or no dispute about it for some 



years. The difiiculty begins when 

 we leave these and take up some of 

 the species and varieties which have 

 much finer marking. 



The most satisfactory method of 

 examination will be found in a pro-, 

 gressive 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 ex- 

 amples with a considerable range of 

 diminishing areolse also. _ In different 

 individuals of the same size there is 

 also often found much difference in 

 fineness of areolation. The gigantic 

 forms of Coscitiodiscus ocidiis iridis 

 found in the Maryland deposits be- 

 come as small as C. radiatus^ and 

 the latter is often found in recent ma- 

 rine gatherings side by side with C. 

 sub tills and of no greater size. 



We are able, therefore, to follow 

 the diminution of undoubted hexag- 

 onal areolje from the greatest of these 

 specimens, where the valves measure 

 .oi6 inch in diameter, till they are 

 scarcely one-eighth as large. Then 

 taking up C. stibtilis with its hexa- 

 gons 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 hexag- 

 onal form to that of round punctae. 

 In this pi^ogression we find that the 

 areola 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 char- 

 acteristic 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 color 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 out- 



