482 PROFESSOR GREGORY ON 
occurring in the Clay Marl of Agina, a bed belonging either to the Chalk forma- 
tion or to the oldest Eocene strata. It seems to be very scarce there, for EHRENBERG 
has figured an imperfect specimen. I found it first rather sparingly in Professor 
ALLMAN’s Lamlash Bay dredging; and, since then, still more sparingly in Mr 
Mixes’s, from the same locality, as well as in several of my Loch Fine dredgings. 
It is obviously a member of the same group as NV. Hennedyi, N. nebulosa, and 
others, with marginal and central striated bands. It is distinguished by its size, 
by the remarkable coarseness of its striation, and by the peculiarity that granules, 
such as form the strize, are scattered over the unstriated space, without regu- 
larity. I have been informed that a form of V. Hennedyi occurs, with a similar 
character, but this I have not seen. I presume it will be easily known by its 
much finer striation, and its smaller size. Though this species is hitherto scarce, 
I have been able to examine a large number of examples, and also to supply 
various correspondents with specimens. 
I avail myself of this opportunity to point out, that we have here an excellent 
example of the occurrence, in the recent state, in our seas, of a species hitherto 
known only as a fossil one. But as the Clay Marl of Agina is the oldest deposit 
in which Diatoms have been detected with certainty, we have evidence that a 
species which is among the oldest of known Diatoms still exists. Nor is this by 
any means an unusual occurrence. In Exrensere’s plate of the microscopic 
forms of this Eocene clay marl (Eocene at least, if not Cretaceous), he figures 
many other forms, all of marine origin ; and all, or nearly all, of which are still 
living species. Indeed, I have seen upwards of three-fourths of these Diatoms in 
the dredgings described in this paper. Among these are Actinocyclus undulatus. 
Coscinodiscus radiatus, Pyaxidicula cruciata, Navicula preetexta, N. Bombus, and 
many other frequent forms. I feel. assured that every form of Diatom found in 
that Clay Marl, still lives in the present seas. And if this be the case with the 
oldest Diatomaceous deposit, it is no less likely to hold good of such as are of 
later date. In the great bed of Richmond, Virginia, which is marine, and said 
to be of the Miocene period, perhaps the most frequent form is Orthosira marina, 
Sm. (olim Melosira sulcata, Kiitz.); a form which I find, as already mentioned, 
very abundant in Lamlash Bay. In the same deposit occurs Coscinodiscus cen- 
tralis, Ehr., a splendid disc, to be described farther on, as occurring in the Clyde; 
and I might multiply similar examples almost ad injinitum. 
Here the question naturally presents itself, Are there any extinct species of 
Diatoms? Strange as it may seem, when compared with what is found to occur 
in organisms of other and higher classes, I believe that this question ought to be 
answered in the negative. 
In the earlier works of EHRENBERG, we frequently meet with species, and even 
with large groups of species, or almost genera, which are stated to be “ fossil 
only,” and which were believed to be extinct. Such forms are Campylodiscus 
