482 PROFESSOR GREGORY ON 



occurring in the Clay Marl of iEgina, 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 

 Miles'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 N. 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 striae, 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 iEgina is the oldest deposit 

 m 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 Ehkenberg'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 radiatas, Pyocidicida cruciata, Navicula prcetexta, 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 infinitum. 



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 



