48 



THE AMERICAN MONTHLY 



[March, 



but the type may be fairly considei-ed 

 a subhexagonal arrangement of areola 

 in the outer lamina of the valve, the 

 walls of these areolte being extraor- 

 dinarily thickened outwardly, making 

 a rough honeycombed surface. The 

 iiiner lamina has its independent sys- 

 tem of very fine circular dots in radi- 

 ating lines, and some of these are 

 seen at the bottom of the bright areolae 

 when the diatom is examined by 

 transmitted light. 



Coscinodiscus Oculus - /ri'dis. — 

 This diatom has also been the subject 

 of discussion, the result of w^hich is a 

 general agreement that its hexagonal 

 markings indicate the walls of true 

 areolae, which lie between two plates 

 in a manner similar to that which we 

 have found in Triceratium favus. 

 Mr. Stephenson reported some years 

 ago {M. M. /., vol. X, p. i) his 

 examination of it in bi-sulphide of 

 carbon, and noted the fact that the 

 inner lamina was sometimes found 

 separated from the outer. He found 

 the hexagons persistently attached to 

 the outer lamina, only the faint out- 

 line being seen on the inner plate 

 vi^hen detached. This latter, how- 

 ever, showed "eye-spots" approxi- 

 mately in the centre of each hexagon, 

 and these consisted of circular con- 

 cavities in the lamina of about half 

 the area of the hexagon. He also 

 noticed teeth projecting from the hex- 

 agon sides upon the upper (outer) 

 lamina, giving, in one position of the 

 objective, the appearance of a row of 

 circular dots around the margin of 

 the areolae. In only one respect vi^ould 

 I difter from his principal conclu- 

 sions. He thought the "eye-spots" 

 open at their centre. Examining the 

 valve as an opaque object with the 

 vertical illuminator and a high power, 

 it will be seen that the film is un- 

 broken, and completely closes the 

 cell in this case as in Triceratium. 

 Near the centre of the eye-spot it is 

 so thin that it cannot be easily de- 

 tected by transmitted light, and might 

 readily be supposed to be wanting. 



Where the outer lamina (on the 



convex sui'face of the valve) meets 

 the hexagon walls, little costae or 

 angle-ribs run down from the walls 

 and extend upon the face of the film, 

 like minute buttresses. These are 

 not found in all specimens of the 

 shell but only in the larger ones. In 

 such a slide as Peticolas' diatoms from 

 Calvert Co., Md., a majority of the 

 valves of this diatom show this feature. 

 The bases of the angle-ribs run into 

 each other making a semi-circular 

 outline, which by repetition makes a 

 scalloped margin, the points pointing 

 inward. This, v^^hen the objective is 

 a little out of focus, gives the appear- 

 ance of a circlet of dots. I have, 

 however, found some examples of 

 very large shells in which the -whole 

 surface of the hexagon is covered with 

 dots as in T. favus. The tendency 

 to support by cost£e a thin film 

 covering an areola, in the manner 

 described, is not confined to Coscino- 

 discus. I have in another place 

 {Am. yourn. Micr.^ vol. iii, p. 127) 

 noticed the same in Isthmia nervosa., 

 and it will be found in other genera 

 also. 



Another fact in the variation of 

 Coscinodiscus is important. In both 

 the Calvert County and the Notting- 

 ham deposits a great many valves 

 will be found in which the hexagonal 

 areolation is not complete. On one 

 edge of the shell it will be found that 

 the areolce have diminished in size, 

 changing from hexagons to circles, 

 and leaving a considerable hyaline 

 space between, instead of being boun- 

 ded by the common hexagon walls. 



Those who may have access to the 

 series of photographs of diatoms 

 made by Dr. Woodward at the Army 

 Medical Museum, will find among 

 them a splendid picture of such a 

 shell. This defect in areolation is 

 usually on one segment of the valve, 

 and the contrast is like that of soap 

 bubbles cro-wded together in one place 

 and separated in another, as seen in 

 experiments in the laboratory upon 

 the tension of elastic films, which 

 may possiblv be more closely related 



