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PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
the typical mammalian dentition there are 44 teeth, divided 
as follows Incisors, 3; canine, 1; premolars, 4; molars, 
3, in each side of both jaws. This number is seldom 
present in recent mammals with two sets; and where some 
are absent, it is generally possible by comparison with 
allied animals to tell which teeth are lost. The teeth are 
implanted in sockets, not anchylosed to the jaws; and the 
articulation is called gomphosis, from the resemblance to 
a nail driven into a board. 
Mr Stewart then gave an account of some of the more 
remarkable types of mammalian dentition, taking the 
various groups in their order, and exhibited in illustration 
a large number of interesting specimens. 
2. “ The Diatoms of the Tay." By Dr Trotter. 
Of the many forms of organic life which come under 
the observation of the microscopist, none prove more attrac¬ 
tive than diatoms. Their almost universal distribution, 
their elegant forms and beautiful and regular ornamentation, 
their great variety, and the ease with which they can be 
procured and mounted for observation and preservation, 
make them at once favourites with all who are in pos¬ 
session of even the cheapest of microscopes. To the 
naturalist, also, they are of great interest as occupying a 
position on the border between animal and vegetable life. 
They are among the most widely-distributed organisms we 
know of, and they play an important part in the economy 
of the universe. They are found in all running waters, and 
their skeletons, or shells, are found in every geological 
formation, from the gneiss that was formed on the 
very eve of creation, to the mud that is being deposited 
by the Tay this afternoon. Mountains are formed 
entirely of their remains, deposited in ages long gone by, 
and the fertilizing mud now being spread over Egypt by 
the overflow of the Nile, is largely composed of them. 
Our own Tay owes its superiority, as a salmon river, 
chiefly to its superabundance of diatoms, which furnish 
food for the many forms of life, which eventually result in 
the monarch of the waters, Anyone walking on the banks 
of the Tay in summer, and examining its pebbles, must have 
observed the thick muddy encrustation which envelopes 
every peeble on its shores. This crust is almost entirely 
composed of diatoms and their gelatinous envelopes, 
mixed with entangled clay and sand, and from it were ob¬ 
tained most of the diatoms which I shall show you to¬ 
night. 
When in the south - west of England this autumn 
I remarked that in the Rivers Urr, Ken, and Dee 
scarcely any of this crust was to be found, and in 
what there was, diatoms were few and small, there being 
not one per cent, of what there are here. Curiously 
enough, though there appears in the Solway to be much 
better marine feeding ground than in the small estuary of 
the Tay, the salmon are very much smaller, a 20 lb. 
fish being chronicled in the local papers as “ another 
monster,” while here a fish of double that weight would 
scarcely secure mention. 
Diatoms may be considered as little transparent cases 
composed of silica, and of various forms, from a cylinder 
or half-globe, to a long, thin, narrow stripe with square 
ends. They are almost always symmetrical, both in figure 
and in the stripes, checks, and hexagons with which their 
surfaces are adorned. Some kinds, instead of being in the 
form of cases, have two parallel flattish ornamental sur¬ 
faces, connected down the centre by a vertical plate, 
somewhat like a portion of a railway plate, and 
giving a transverse section resembling in appearance 
a dumb-bell. Diatoms appear to multiply chiefly by 
self-division, as many of the lower forms of life do, 
but, though the fact is not yet established, they appear 
also to increase by spores, as we find many sizes of the 
same variety in the same particle of mud, some perhaps 
l-100th, and some less than l-1000tb, of an inch in length. 
We find also that a small stream has to trickle over a rock 
for only a few days, when the rock becomes covered with 
countless millions of these minute organisms, a circumstance 
only to be accounted for by supposing the germs to have 
been deposited there by the water. 
The waters of the Tay and its tributaries are very rich 
in diatoms, and I have already picked out and mounted 
over a hundred distinct forms. There is great difficulty 
in naming most of them, as very few correspond altogether 
to the typical forms. Indeed, they seem to differ in form 
in almost every tributary stream, and even in the Tay 
itself changes of form are found every few hundred yards. 
The diatoms of high water also are different from those of 
low water, and those of the stream from those from still 
water. A great many well-known British forms, how¬ 
ever, are found, as well as other forms which I have not 
been able to identify. Diatoms are divided into two princi¬ 
pal sections,—those which have no central nodule or true 
median line, and those which have a median line and a 
true central nodule,—both of which are well represented, 
[After exhibiting and describing some of the species 
found in the Tay, Dr Trotter referred to those which 
occur in a fossil state in the clay beds of the district, as 
follows.] 
Strange to say, their shells are as beautiful and per¬ 
fect in every respect as if they had lived and died 
