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TET 
Sength of one’s tetlier; to speak or act witji 
as much freedom as circumstances will ad- 
mit. 
TETHYS, in natural history, a genus of 
the Vermes Molhisca class and order : body 
detached, rather oblong, fleshy, without pe- 
duncles y mouth with a terminal, cylindri- 
cal proboscis, under an expanded mem- 
brane or lip ; two apertures on the left side 
of the neck. There are two species, viz. 
T. leporina, which inhabits the Mediterra- 
nean, and T. fimbria, found in the Adriatic. 
TETRACERA, in botany, a genus of 
the Polyandria Tetragynia class and order. 
Natural order of Rosaceoe, Jussieu. Essen- 
tial character j calyx five or six-leaved ; co- 
rolla four or five-petalled ; filaments widen- 
ing above; and anther bearing on each 
side; capsules four, opening on the side; 
seed arilled at the base. There are twelve 
species. 
TETRACHORD, in the ancient music, 
a concord consisting of four degrees or in- 
tervals, and four terms or sounds ; called 
also by the ancients diatessarron, and by us 
a fourth. 
TETRAD YN AMIA, in botany, the name 
of the fifteenth class in the Linnaean system, 
consisting of plants with hermaphrodite 
flowers, having six stamina, four of which 
are longer than the rest. There are two 
orders in this class, viz. the siliquosm, those 
that have long pods, as stocks, rockets, &c.; 
and the siliculosae, or those that have short 
round pods, as scurvy-grass, candy-tuft, 
&c. 
TETRAEDRON, or Tetrahedron, in 
geometry, one of the five regular or plato- 
nic bodies or solids, comprehended under 
four equilateral and equal triangles. 
It is demonstrated by mathematicians, 
that the square of the side of a tetraedron 
is to the square of the diameter of a sphere, 
wherein it may be inscribed, in a subsequi- 
lateral ratio: whence it follows, that the 
side of a tetraedron is to the diameter of 
a sphere it is inscribed in, as v/ 2, to the 3, 
consequently they are incommensurable. 
TETRAGONIA, in botany, a genus of 
the Icosandria Pentagynia class and order. 
Natural order of Succulentm. Ficoidete, 
Jussieu. Essential character : calyx three 
to five-parted ; petals none ; drupe inferior, 
inclosing a nut from three to eight-celled. 
There are eight species, chiefly natives of 
the Cape of Good Hope. 
TETRAGYNIA, in botany, the name of 
an order in certain classes of the Linnsean 
system, consisting of plants, which, to the 
VOL. VI. 
TEt 
classic character, add the circumstance of 
having four styles. 
TETRANDRIA, in botany, the name 
of the fourth class in the. Linnman system, 
consisting of plants with hermaphrodite 
flowers, which have four stamina of equal 
length. In this last circumstance consists 
the main difference between the tetrandria 
and the didynamia, in which the four sta- 
mina are of unequal length, two of them 
being longer than the other two. There 
are three orders in this class, founded upon 
the number of styles. 
TETRANTHUS, in botany, a genus of 
the Syngenesia Polygamia Segregata class 
and order. Natural order of Capitatas. Ci- 
narocephalae, Jussieu. Essential character : 
calyx common, four-flowered ; perianth 
proper, one-leafed ; seeds crowned. There 
is but one species, viz. T. littoralis, an an- 
nual plant, and a native of Hispaniola. 
TETRAO, in natural history, a genus of 
birds of the order Gallium. Generic cha- 
racter; near each eye a spot which is naked, 
or papillous, or slightly covered with fea- 
thers. Birds of this genus, which, accord- 
ing to Gmelin; comprehends the grouse, the 
partridge, and the quail, follow the dam 
immediately on being hatched, and before 
the shell is wholly detached from them ; 
their bill is strong and convex, and their 
flesh and eggs form an exquisite repast. 
There are seventy-three species, of which 
the following are best deserving of notice. 
T. urogallus, or the cock of the wood, is 
of the size of a turkey, and is found from 
Russia to Italy, preferring the elevated 
and mountainous parts of temperate coun- 
tries, as it delights in a cold temperature. 
Its eggs are deposited on moss, and when- 
ever left by the female, who is unassisted in 
the process of incubation, are covered over 
with leaves. The males and females live 
separate, except during the months of Fe- 
bruary and March. Their food consists of va- 
rious plants and grains, and of buds of trges. 
The seeds of the pine and fir they are par- 
ticularly fond of. The sound of the male 
resembles not a little the whetting of a 
scythe. These birds are in high request 
for the table, and are sometimes sent from 
Petersburg to London, in a very rigorous 
winter, arriving, it is said, in good condi- 
tion. 
T. tetrix, or the black grouse, is larger 
than a common fowl, and abounds in the 
British islands, particularly in the northern 
districts. In winter these birds shelter 
themselves in low situations. On the returH 
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