Greville, on New Diatoms. 
45 
accountable as it may seem, these successive experiments 
often brought species to light previously unobserved, and 
sometimes never seen afterwards, the same cautious system of 
investigation being pursued in all cases. It may be remarked 
that, as a general rule, those examples of the deposit which 
are of a chalky or light density are poor in diatoms, while 
they are sometimes rich in polycystins ; but it by no means 
follows that the converse prevails. This Mr. Johnson^ Mr. 
George ]S[orman_, and myself, have ascertained to our cost. 
To the courtesy of his Excellency James Walker, Governor 
of Barbadoes, I have been indebted for a box of specimens 
from various localities in the island, and Dr. Mouat most 
kindly supplied me with another large collection ; and 
although a number of these came from the Cambridge estate, 
and bore a close outward resemblance to the solitary precious 
sample received by Mr. J ohnson, not one of them repaid the 
trouble of cleaning. Mr. George Norman likewise examined 
an equally extensive series of specimens transmitted to him 
last year, and very promising in appearance, but with no 
better result. Many specimens, when prepared for examina- 
tion, are found to be composed, as far as organic remains are 
concerned, of the mere pulverized debris of diatoms and 
polycystins ; others may contain debris less broken up, with 
possibly a few entire discs of one or two species. Others, 
again, are more productive, and the most rich in every sense 
are those in which the heavier diatomic forms prevail ; a fact 
which seems to be connected with the law of gravitation at 
some period in the history of the formation of the deposit. 
But some additional causes seem to have been at work rela- 
tive to the polycystins ; for while diatoms occur most abun- 
dantly in certain of the more compact and heavy parts of the 
deposit, polycystins are more copious and perfect in some of 
the lighter parts ; and, although the two orders are more or 
less intermingled, those parts in which one order is developed 
most abundantly, both in number of species and perfection of 
structure_, are deficient in the other. The deposit is said to 
be very extensive, and to be of great thickness. It is to be 
regretted that no memoranda have accompanied the speci- 
mens transmitted to this country, regarding their relative 
position in the deposit, &c. Nothing but a careful examina- 
tion on the spot, along with a diligent use of the microscope, 
will suffice to clear up the difficulties we labour under on the 
whole subject. 
Coscinodiscus splendidus is exceedingly similar in general 
appearance to Cresswellia superba ; so similar, indeed, that if 
the spines of the latter happen to be out of focus, the one might 
