106 



THE AMEKICAK MONTHLY 



[June, 



fracture follows the dots, and the hy- 

 aline parts of the valve show^ the pink 

 color, so that both lines of proof still 

 combine to show the dots to be the 

 weak and thin places in the valve. A 

 beautiful example of the latter sort 

 is Mastogloia angulata^ Grunow, 

 which is not uncommon in Long 

 Island Sound, and is found along the 

 whole Atlantic coast. The shell is 

 broad ovate, somewhat cuspidate, of 

 smooth even thickness, and the punc- 

 tae are arranged in oblique rows. 

 With a medium power the effect is 

 that of a delicate cross-hatching, much 

 like that of Pleurosigma angulattim. 

 With a high power the dots are well 

 separated, and, except as to arrange- 

 ment, their appearance is similar to 

 those of N. sculpta. As in the disc 

 forms, the diminishing size of the 

 areolae brought us gradually very near 

 to the fine lines of Hyalodiscus sub- 

 tilis^ so among the naviculae we make 

 a similar approximation to the delicate 

 marking of the pleurosigmas. 



The use of the vertical illuminator 

 upon these diatoms is hardly less de- 

 cisive in support of the conclusions I 

 have drawn than in the case of the 

 coscinodisceae. A smooth surface 

 dotted with tiny bubbles is the char- 

 acteristic appearance of the shell, and 

 these bubbles in the larger kinds can- 

 not be distinguished from those which 

 we have found in the disc forms, be- 

 ginning with examples from the Not- 

 tingham earth where the hexagons 

 and round areolae were found side by 

 side upon the same valve. We may 

 even take a step in advance. In 

 Peticolas' slides of Richmond and 

 Petersburg earths there are numerous 

 examplesof a coarse form oi P. angu- 

 latum^ var. Virginicuni^ in which 

 the marking in the middle of the valve 

 is coarser than at the extremities. On 

 dry specimens of this shell a high 

 power used with the vertical illumi- 

 nator will separate the dots sufficient- 

 ly to show a surface hardly to be dis- 

 tinguished from that of Mastogloia 

 a?igulata which I have noticed above. 

 It is a smooth film in which the mi- 



nute bubble-like dots have the same 

 character and differ only in size from 

 those in Actinocyclus^ or in the 

 coarser smooth naviculae. In some 

 broken specimens, also, the line of 

 fracture could be traced through the 

 dots. 



In Stauroneis ptdc/ie/la the areolae 

 are much longer in proportion to 

 their width, and are contracted at the 

 ends so as to take the ' oat-shaped ' 

 appearance by which they are com- 

 monly known. There is here found 

 a difference in the appearance of the 

 concave and convex sides of the 

 valve, the former presenting the 

 areolae more nearly as rectangles, 

 and the latter giving moi-e of the 

 spindle shape. This is analogous to' 

 the difference noted in other genera, 

 the outer view of hexagonal markings 

 being usually nearly ciixular, whilst 

 the inner shows the angles more 

 clearly. In Epit/iemm turgid ay as 

 found in MoUer's preparation from 

 the Sodertelge mud, the frame-work 

 of the shell is a nearly rectangular 

 lattice, the areolae showing all the 

 peculiarities of light which have been 

 described in Navicula lyra^ and the 

 fracture often shows the ends of the 

 frame-work sticking plainly out be- 

 yond the sides of the adjacent dots. 

 The same may be seen in the elon- 

 gated areolae of Amphora ovalis of 

 the larger varieties. In Cocconeis 

 splendidum the hexagons are as dis- 

 tinctly formed as in Cosci?iodiscus^ 

 and in C. scutellum the areolation 

 varies froin coarse to fine with the 

 diminishing size of the valves, giving 

 a series analogous to that which are 

 found in Coscinodiscus subtilis^ and 

 one in which the fracture is as plainly 

 through the dots, whilst the evidence 

 of relative thickness or thinness of the 

 silex from the color is as we have 

 found it in other cases. 



But to complete the list of species 

 in which I have found the tests of 

 fracture and of color supporting the 

 theory of areolation of the diatom- 

 shell, and contradicting that of solid 

 spherules, would be too much like 



