Gregory on Diatomaceous Earth. 
95 
Himantidmm gracile, Kiitz. 
bidens, W. Sm. 
pectinale, Kiitz. 
„ arcus, Kiitz, 
major, W. Sm. 
„ undulatum, Ealfs. 
Tabellaria frustrata, Kiitz. 
„ ventricosa, Kiitz. 
Epithemia turgida 
gibba 
Eunotia gracilis 
„ retrorsum 
Diadema 
Synedra capitata 
biceps 
Fragillaria capucina, Kiitz. 
Orthoseira viridis, W. Sm. 
„ onchalcea, W. Sm.. 
Gomphonema Vibrio 
„ capitulatum 
Amphora ovalis 
Staiironeis Phoenicenteron 
„ gracilis 
linearis 
„ anceps 
Oymatopleura elliptica 
„ apiculata 
Cocconeis Thwaitesii 
„ Placentula 
Surirella Brightwellii 
„ biseriata 
Cymbella Helvetica 
„ maculata 
„ sativa 
„ affinis 
cuspidata 
It will be perceived that Mr. Smith has found, in the speci- 
mens sent to him, 59 species of fresh-water Diatomaceae. 
As 1 had made sketches of all those forms which I could not 
name, I was easily able to identify Mr. Smith's species. I 
have stated that I had distinguished about 65 forms. I 
believe that some of these were side-views of species un- 
known to me at the time, and others, in all probability, 
accidental varieties. But I also think it probable that there 
may be a few species in the deposit which do not occur in 
the portion seen by Mr. Smith. At least, I am quite certain 
that that portion differs remarkably in some points from that 
which I had under examination at the same time. For ex- 
ample, in Mr. Smith's specimen, of which he kindly sent me 
two slides as I had not tested it myself, I find that there are 
numerous and fine loricae of Epithemia turgida — a species 
which I had indeed observed in mine, but which I had found 
remarkably scarce. 1 have reason to think that hardly any 
two specimens will be found exactly to agree, and it is quite 
natural that different parts of the deposit should differ in the 
prevailing forms. Among the forms which I thought I had 
observed, but which Mr. Smith did not meet with, are Melo- 
siera distans, and possibly M. nummuloides ; Eunotia 
Triodon^ and E. Pentodon ; possibly E. fabra, and one or two 
more. But most of these, if they do occur, are very scarce ; 
and therefore I do not venture to add any names to Mr. 
Smith's list until I shall be confirmed by him or by some 
other experienced authority. There are several other forms, 
also doubtful, which I thought I had seen, but I need not 
name them. 
The Mull earth is characterised by' several peculiarities. 
First, by the abundance of very fine specimens of the Navi- 
