86 Dr. Gregory, on the Post-tertiary 
so far as I have been able to complete the examination of 
them. But several forms still remain, which must be reserved 
for more full examination. 
It will be seen that, including the additional list of pre- 
viously described forms, and those now first described, the 
number of species already found in this remarkable deposit, 
which in my former Paper extended to 234, now amounts to 
304 ; a number, so far as I am aware, very far surpassing 
anything elsewhere observed, and nearly three times as great 
as that of the species in the estuarial mud of the Thames, as 
described by Mr. Roper, which is almost the only deposit, 
rich both in fresh-water and marine forms, hitherto fidly 
described. The deposits and gatherings described by Ehren- 
berg have generally been either marine or fresh-water alone. 
I have shown the probable cause of the mixture of marine and 
fresh-water forms, but I cannot suggest any satisfactory 
explanation of the unusual accumulation of species, and can 
only conjecture that the deposit was formed during a very 
long period, in the course of which waters from difi*erent 
quarters may have at different times been carried to the 
spot. 
But the Glenshira sand or mud is even more remarkable 
for the large proportion of new or undescribed forms, than for 
the aggregate number of species. In this and the former 
part of this communication, I have described and figured 
upwards of sixty forms, all undescribed before, so far as my 
knowledge extended. This is exclusive of several, which are 
probably varieties of known forms, as well as of eight or ten 
which I have elsewhere figured as fresh-water species, found 
by me in recent gatherings, and either new to science or new 
as British forms, which also occur in this sand, and several of 
which I first observed in it. It is also exclusive of a good 
many new forms observed also elsewhere by other naturalists. 
Of the sixty forms just alluded to as new, only a very small 
number have as yet occurred elsewhere, although most of 
them are frequent in our deposit. From this I conclude that 
a large number of unknown forms will still repay the observer 
who looks for them, especially among marine species. (This 
conclusion is confirmed by the recent gathering above men- 
tioned, in which I find, not only about 40 of the new 
Glenshira forms, but a number of species entirely new, of 
which I gave a short preliminary notice to the Botanical 
Section at Cheltenham.) A very extensive acquaintance with 
our fresh- water Diatoms, of which I have minutely searched 
gatherings from not less — probably many more — than 500 
British localities, enables me to say that new forms now occur 
