HENDRV^ ON HYALODISCUS SUBTILIS. 181 



complimental to that previously exhibited only iu part^ and 

 then with a field of decussation or intersection but limited in 

 degree. 



When surveying- an entire shell, well calculated for observa- 

 tion^ the following characteristic features present themselves, 

 — the two decussating and diverging portions of the threefold 

 series of lines are to be found only in the direction of the 

 axis of illumination, and also at the opposite extremity of the 

 said axis, while the radiating portion of the series will be found 

 at either extremity of a line at right angles to the foregoing, 

 [id est), that each alternate quadrant of the circle is the seat 

 of decussating or otherwise radiating portions of the series, 

 the decussating always being on the line of axis of illumina- 

 tion, and the radiating at right angles to these, and upon re- 

 volving the slide through an arc of 90 degrees, the previous 

 radiating now become decussating, and the previously de- 

 cussating now become radiating, and so ringing the clianges 

 throughout the disc ; the same phenomena are consequent 

 upon manipulation with any fragment howsoever, possessing 

 a readily visible striation, as well as upon the entire shell ; 

 indeed, for ordinary observation, I have found some fragments 

 of greatest interest, — upon two or three of which in my 

 possession the markings are so vividly displayed through 

 peculiarity of shell structure, that with a Dallmeyer's -pV inch 

 objective of 165° angle of aperture, I am easily enabled to 

 command any measure of amplification of the same, within 

 the limit of four thousand diameters, [id est, 120 x by 30 

 (eye-piece objective) = 3"60() + axis extension := 4000, a fact 

 which I hereby employ to signify the value and brilliancy of the 

 shells in question, concluding hence that with choice of such 

 a range of magnitude and distinction, every existing feature 

 ought to be fully and fairly elicited, — somewhat warranting 

 the conviction, that the previous representations of authors 

 are exaggerated; and whatever may constitute the normal shell 

 structure, the direction of illumination plays no unimportant 

 part in the display of varied phenomena; nevertheless, there 

 exists undoubted laeauty, well worthy of the keenest research. 

 Radiating lines, if these were uninterruptedly continuous 

 from the centre to the circumference, they must necessarily 

 diverge, and produce wider interspaces towards the border of 

 the shell, which is not apparently the case ; for some speci- 

 mens exhibit a peculiar mottled appearance, occasioned pro- 

 bably (as seen evidently on other discoidal diatoms) by the 

 insertion of other shorter lines, shortening still as their suc- 

 cessive insertions approach the periphery, thus presenting a 

 series of zones, and so occasionina" the markings at the peri- 



