NOTES AND MEMORA-NDA. 39 



under which the spherules are formed prohably differ as decidedly 

 from those surrounding the growth of the diatom, as does the manu- 

 facture of shot from a natural formation of lead. As has been shown 

 by Mr. Slack, a very slight change in the conditions of the experi- 

 ments is often followed by a difference in the character of the films 

 produced." After some further observations the writer advances to 

 describe the different preparations of diatoms examined by him : 



Coscinodiscus occulus iridis. Best illumination, at a north window, 

 from white clouds. The specimen very large, and the two plates 

 separated. The inside layer perforated with circular openings; no 

 film detected. The plate is thickest on the border of the openings, 

 and the hexagonal network of the outside plate lies in the depressions 

 between and everywhere around these thickenings of the inside plate. 

 The outside plate is fui-nished with a thin film over each hexagonal 

 areola, with a fine angular branching network extending out from 

 the coarse hexagonal i*idges into the films to strengthen them, and is 

 coarsest nearest the ridges, and is very fine, or, in some cases, entirely 

 diaphanous in the central parts of the areolae. The film is seen per- 

 fectly along the line of fracture, which always passes through the 

 depressions. The thick portion next the holes, in the inside plate 

 described above, occasionally extends beyond the line of fracture, 

 owing to its greater strength. Distinct shadows of the sides of the 

 hexagons are observed as the mirror is thrown to one side or the 

 other.* 



Aulacodiscus SamcBnsis. Structure the same as Coscinodiscus. 



Terpsince Americana and T. musica. The structure of both the 

 same. A distinct angular open network, porous almost like a sponge. 

 As the objective is lowered the outside markings gradually become 

 indistinct, as the lower and finer ones of the same general character 

 come into focus. 



Epithemia Hyndmannii, S. and E. turgida. An outside plate with 

 hexagonal depressions and network, strengthened by internal trans- 

 verse ribs. It breaks through the depressions and leaves well-marked 

 projecting points. 



Campylodiscus. The various species of this genus are seen with 

 perforated plates and the line of fracture running through the 

 holes. 



Trinacria regina. Perforated plate, with fracture through the 

 holes. 



Cymbella. Very large members of this genus, observed in hundreds 

 of specimens, with structure resembling grating, and fracturing 

 through the depressions, leaving the points of the grating distinctly 

 projecting. 



Gomphonema geminatum. This shell is very instructive, for the 

 ribs radiate and branch, diminishing in strength toward the margin in 

 the best way to make the shell strong and light. The line of fracture 

 runs through the so-called beads in any direction. 



Stauroneis Stodderii. A peculiarly and very strongly marked shell. 



* Compare with the careful and very accurate observations of Mr. J. W. 

 Stephenson, ' Monthly Microscopical Journal,' vol. x., p. 1. 



