224 
PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
©r not, they are fouDd in the clays of fresh, brackish, and salt 
waters alike, in the mud of the River Amazon 1200 miles from 
the sea, and in the sand at the bottom of the Indian Ocean, and 
in some alluvial clays they are the only organisms that occur. 
The spicules of a considerable number of sponges, however, are 
of exactly similar appearance. 
An undoubted sponge spicule, however, is also found in 
this lowest clay, but is much scarcer. It somewhat resembles 
one-half of the other, but is more slender and less tapered, with 
a smaller portion furnished with a head attached at an angle of 
about 5 deg., this smaller piece having usually a few little 
protuberances on it. It looks very like a pin, slightly bent 
about l-6th of its length from the head, and is hollow. 
A very pretty diatom, but oue which is very awkward to mount, 
occurs in this bed of clay, Campylodiscus Clypeus, of which I have 
been unable, from want of proper cells, to preserve a perfect 
specimen, the coyer-glass breaking them in a day or two by its 
weight. This diatom is an oval plate of silica bent till the out¬ 
line of its edge forms a semi-circle, and when seen in front 
looks like a circle with a portion cut off. 
Another rather common diatom in this clay is a little round 
brown-coloured one, of which there are two varieties, the one 
having a round hole in the middle, and the other having none. 
They appear to be the upper and lower halves of a little circular 
box, each half being about one-third of a globe. I have not 
been able to ascertain the name, but they are found in recent 
clays also. They show no markings, with the exception of a 
ring round the edge (seen with a l-14th objective;, though the 
brown colour points to the existence of such. 
A very pretty little diatom, apparently a Surirella , is very 
common, but is difficult to pick out, it is so thin and fragile. It 
is navicular or boat-shaped, but has no central raphe or nodule. 
It is a thin transparent plate, without any central markings, but 
with a row of little ridges, one-fourth of its diameter in length, 
forming a sort of border rouud the edge. 
All these are brackish-water diatoms, but among them turns 
up a very familiar fresh-water diatom, Gomphonema geminatum, 
differing from the ordinary forms of the Tay by being broader 
and shorter. This is one of the finest but commonest of all. 
This diatom has a habit of turning up In all sorts of unexpected 
places, and last summer 1 found it in some salt-water mud near 
the Mull of Galloway, the nearest fresh-water being over a mile off. 
It would appear from the above that this clay was deposited 
from brackish water, but whether the organic remains were 
once alive where found, or whether they were carried down by 
the stream from a higher source, it is impossible to tell. 
Above this blue clay lies a bed of sand about 10 feet thick, in 
which I found no diatoms, and above that a thin bed of clay 
with remains of vegetable substances intermixed. In this 
various forms of Pleurosigma were very numerous, the most 
plentiful being a very slender and fragile one without raphe or 
central nodule, but with a sort of milled edge all round it. 
Some of the ordinary forms,— Pleurosigma angulatum, for in¬ 
stance,—with both raphe and nodule, were tolerably common, 
while an occasional specimen of Pleurosigmata lanceolatum 
turned up, and also a few fragments of Bacillaria. 
A fine large form of Coscinodiscus was also got in this clay. It 
was perfectly flat, with a thin transparent ring round it, and 
the whole surface covered with fine honeyeombings, without 
any appearance of pattern, or any central marking. This form 
is common in many of the clays of the Perth basin. A curious 
little Coscinodiscus was also found, which I had not seen before, 
and could not find a name for. It was a deep little box with a 
broad rim, the middle filled up with honeyeombings arranged 
regularly round a central hexagon. 
Another rather common diatom in this clay is a little Surirella, 
known as Surirella plicata, a broad-pointed oval disc, with 
a beautiful narrow-beaded edge, and three transverse bands 
across it, with portions of other bands at the two points. Two 
curious objects were also found in this clay looking like 
interlaced threads of glass. They seemed to he the edges of 
the box formed by some diatom, but the diatom itself did not 
turn up. A form of Surirella plicata was also found which 
was not pointed, possibly one in a transition state. There 
were also plenty of sponge spicules, broken and entire, and a 
pretty little marine diatom, somewhat like a Kavicula, hut 
nearly as broad as it was long; several Naviculoe, both fresh 
and brackish-water forms; Pinnularia, Melosira , Gomphonema, 
Epithemia, and other distinctly fresh-water diatoms, but all 
differing more or less in form from those at present found in the 
Tay. It would appear from the above that at the period these 
were deposited the water had been shallow, and the adjacent 
banks had been overflowed by the river, which had deposited 
vegetable matter and fresh-water diatoms there at flood tide. 
Above this is a thick bed of sandy clay largely intermixed 
with vegetable remains, portions of reeds, sedges, and willows, 
birch stumps and trunks of oak trees. It is very compact, and 
when dry resembles soft carboniferous sandstone in appearance. 
In this no diatoms were found—even the universally-present 
Spongiolithis being absent. Resting on this is a layer of clay, 
with little admixture of sand, but containing vegetable debris, 
and a few crystals of smoky quartz. In this none but fresh¬ 
water diatoms were discovered, such as Pinnularia (seme of 
them very large), Eunotia, Epithemia, Mavicula, Melosira, and 
our constant friend Spongiolithis, but not a single brackish 
or salt-water specimen, showing that for some considerable 
period the sea had receded, leaving the channel of the river 
much higher than at present, aud quite abeve the influence cf 
the tides as far as the mingling of the waters was concerned. 
This, again, is covered by the Friarton brick clay, a very 
saudy material, containing very few diatoms and a few frag¬ 
ments of Spongiolithis, those which are found being brackish- 
water forms. One of these resembles the halves of two Pleuro- 
sigmee joined together in a line, with the curls at the ends both 
pointing to the same side, and a stricture in the middle—a 
curious-looking object, the name of which I have been unable 
to ascertain. 
The next higher clay to which I could obtain access was that 
in Priory Place, where, at the time the sand beach of Craigie 
was formed, the water must have been about 15 feet deep. 
This clay is very sandy and much impregnated with carbonate 
of iron, hut nothing was found in it but a few sponge spicules. 
