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three times as large as has hitherto been observed in any other de- 
posit. Of these, he mentioned on the present occasion about 140, of 
which about 120 were known and admitted already as British species. 
The following species were new to Britain, but figured by conti- 
nental authors : — Epithemia gibberula, Eunotia depressa, Kutz., E. 
Camelus, E. bigibba, Kuéz., Himantidium exigerum, Breb., Navicula 
Trochus, Lhr., N. levissima, Cocconema gibbum. Figures of these 
species were exhibited, and in the case of Eunotia bigibba nine re- 
markable varieties were figured. 
The following species were described and figured as new to sci- 
ence :—Eunotia incisa, Pinnularia latestriata, P. undulata, P. exigera, 
P. tenuis, P. parva, Stauroneis rectangularis, Navicula apiculata, 
Smith, Cymbella tumens, Gomphonema Brebissonii? G. Hebridense. 
Professor Gregory then directed attention to the remarkable varia- 
tions of a form which had been referred by Mr. Smith to his Pinnu- 
laria divergens, but, as found in the Mull earth, had more than double 
the number of striza which belong to that species. It occurs in 
several very striking varieties, and the conjecture was thrown out, that 
it is in reality not P. divergens, but a distinct species, including P. 
stauroneiformis, P. interrupta, and P. mesolepta of Smith, as well as 
what he took for P. divergens in the Mull deposit. 
Some remarks were then made on the value of generic and specific 
characters in the classification of the Diatomacee. It was shown 
that the genera Cymbella and Cocconema, Eunotia and Himantidium, 
seem to be respectively separated on insufficient grounds. In regard 
to specific characters, it was pointed out that while certain species 
vary almost ad infinitum, others exhibit a remarkable degree of per- 
manence. Examples of the former are Eunotia triodon, EK. bigibba, 
Himantidium bidens, H. undulatum, Pinnularia divergens, and others ; 
of the latter Eunotia tetraodon, E. Diadema, Navicula Trochus, N. 
serians, N. rhomboides, Pinnularia alpina, and others. It was shown 
that these very characters of permanence and variableness might be 
usefully employed as specific characters, and that viewed in this light, 
both of them afford strong proof of the real existence of species as 
natural divisions. 
Professor Gregory announced that he had still several new forms 
found in this deposit to describe on some future occasion, and that he 
was engaged in the study of other deposits; in all of which he had 
already found species hitherto overlooked, and among these several 
of the new forms above named. 
