96 
Gregory on Diatomaceous Earth. 
culaceae, especially of the genera Phmularia (14 species), 
Navicula (6 species), and Stauroneis (4 species). There are 
many splendid individuals of Pinnularia major (some l-50th 
of an inch in length), oblonga, virides, divergens, and others ; 
and a few, but these very fine ones, of P. lata, and of the rare 
and beautiful P. alpina. Navicula rhomboides and N, 
seriajis are particularly frequent and fine, as is also Stauroneis 
Phcenicenteron. 2ndly. It is characterized by the abundance 
of Cymhellos of which there are 5 species. 3rdly. There is 
a remarkable development of the Eunotice, as Eunotia 
Tetraodon, E. Diadema, Himantidium Arcus, H. hidens, and 
the 4 other Himantidia and Epithemia turgida. 4thly. There 
is a great abundance of Tabellaria fenestrata in every stage 
of development, some specimens being 10 or 12 times as long 
as others, but not broader, and of T. ventricosa which, how- 
ever, occurs almost always short. 5thly. There is a remark- 
able abundance of fine specimens of Gomphonema coronatum, 
and fine individuals of G. acuminatum also occur. The 
genera Amphora, Cyniatopleura, Cocconeis, Surirella, and 
Nitzschia occur less abundantly, and in some cases are 
very scarce. Fragilaria capucina, Kiitz., Orthoseira viridis, 
W. Sm., and 0. ouchalcea, W. Sm., are abundant, as is Synedra 
biceps. I have observed the variety /3 recta, Kiitz., of this 
species. 
Besides the 59 species named by Mr. Smith (and I would 
again remind the Society that the names in the above list are 
those of Mr. Smith's daily expected Synopsis), there is one 
form, to which I directed his attention, and which he cannot 
with certainty refer to any known genus. This form is 
abundant in all specimens of the earth, and is therefore an 
additional characteristic of it. It varies from l-600th to 
I -470th of an inch in length, and has usually the form of a 
piano convex lens, with two notches near the ends of the 
plane or very slightly concave side. It is broadest in the 
middle, and has sharp apices (fig. 1). At other times 
the apices are less sharp and the ends broader (fig. 2). It 
is finely cross striated, and Mr. Smith has ascertained the 
number of striae to be 44 in 1-lOOOth. It requires a very 
good glass to make out the striae, and it is possible that this 
form, from its abundance in the Mull earth, may be found 
available as a test object. For a long time I could not make 
out the striae (although I felt sure of their existence from the 
resemblance or aspect to other forms known to be striated) 
with a glass which had sufficed for all the other forms. But 
with a first-rate object glass, and good management, the striae 
may be shown and counted. It is possible that this form 
