N. E. GREEN ON THE SURFACE MARKINGS OF DIATOMS. 233 



forms were slightly hexagonal, though still maintaining the crater- 

 like form. (See fig. 1.) Biclclulphia, Triceratium, and a form 

 allied to Coscinodiscus were examined in succession, and all yielded 

 the same characteristic markings. (See figs. 2 and 3.) 



A slide of Isthmia, mounted in the usual manner for viewing by 

 transmitted light, was then placed on the stage and examined by 

 light reflected from the mirror ; but no evidence of the existence of 

 the craters could be obtained. True, there were markings in the 

 same places, but they appeared as projections rather than hollows, 

 and no arrangement of transmitted light gave any hint of their true 

 form. 



The next object submitted to the test of side illumination was P. 

 Hippocampus. A drop of water containing several specimens of 

 this and other diatoms in the living state, was placed on glass, and 

 the water allowed to evaporate. Here an entirely different kind of 

 surface was observed. The lime light brought out most distinctly 

 the bead-like character of its markings ; they stood out in bold 

 relief, like rows of Indian corn. (See fig. 4.) And this bead-like 

 surface was seen also on Cocconema, and a few others on the slide. 

 (Fig. 5.) 



By the kind attention of Mr. Wheeler, of Tollington Road, I 

 have since been supplied with many specimens of Navicular (mounted 

 dry on slips of glass and uncovered). Of these, P. Acuminatum, 

 P. Formosum, P. Elongatum, P. Balticum, and P. Angulatum have 

 all given the same results, though the size, form, and projection of 

 the beads differ considerably in the various kinds. 



In Formosum the beads are smaller than in Hippocampus, and 

 by no means so boldly relieved ; indeed, at the edges of the valve 

 there is a considerable flatness in their shape. In the case of An- 

 gulatum, when first submitted to the test of side illumination, the 

 strong resemblance to its appearance when examined by transmitted 

 light was somewhat surprising. There were the usual file-like 

 markings, and even the hexagonal form was intimated ; and it was 

 only after the most careful adjustment of the light that a beaded 

 appearance was brought out. A most interesting fact was observed 

 in a valve of Angulatum, from which the coating of beads had been 

 partly removed by abrasion, leaving portions still adhering like bits 

 of bark on a tree. At the edges of these, the beaded form was more 

 easily perceived than in the uninjured portions ; there was evidently 

 a thickness in the coating of beads, and this thickness was decidedly 



