190 
PROCEEDINGS OP THE PERTHSHIRE SOCIETY OP NATURAL SCIENCE. 
of two varieties of sea-urchin, starfish, whelks, limpets, [ 
periwinkle, dog-whelk, &c. The front teeth serve to pull 
the shell-fish from the rocks to which they are generally 
adherent; the back ones, moved by the powerful masti¬ 
catory muscles, crush the shells. In the sharks and rays 
(ElasmobranchiiJ, the skeleton is cartilaginous; the teeth 
can therefore never be anchylosed to the jaws. They are 
arranged in several rows along the margins and posterior 
surfaces of the jaws, and are implanted in a dense 
fibrous membrane. During life this membrane slides 
forwards over the edge of the jaw, carrying with 
it the rows of teeth. As soon as the foremost row 
reaches the outer edge it drops off, but by this time 
the one behind has come into its most effective posi¬ 
tion. New rows are continually developed to make good 
the loss. This perhaps explains why fossil sharks’ teeth 
are so common, other parts of the animal being seldom 
found. Some of these fossil teeth are of enormous size, 
and if they can be taken as a guide to the size of the 
animals, former sharks must have been as large as whales. 
The teeth are mostly triangular; some being serrated. In 
the rays they are flattened, and carry a sharp point, the 
rows resembling mosaic pavement. The saw-fish (PristisJ 
resembles the above in its oral dentition, but projecting 
from the upper jaw is a broad spatula-shaped weapon, 
the rostrum, armed along each side with large teeth. 
These teeth are peculiar for their mode of attachment, 
structure, and growth. The rostrum sometimes attains a 
great size. 
Amphibia .—The teeth are fewer than in the fish, and 
placed only on one or two bones, There is a continual 
succession. In the frog, teeth are found on the upper jaw 
and the vomers. The toad is edentulous. The labyrin- 
thodon, a gigantic extinct amphibian, shows a labyrinth¬ 
like arrangement of the dental tissues, from which the 
name is derived. 
Reptiles. — In the tortoises and turtles (Chelonia) 
teeth are absent, the jaws being covered with horn. 
The serpents (Ophidia) may, for convenience, be divided 
into two classes, the poisonous and non-poisonous. In 
the former the teeth are placed on the upper and lower 
jaws, the palate and pterygoid bones, forming 4 rows 
above and 2 below. They are sharp recurved cones, an¬ 
chylosed to the bones ; and are only useful for seizing and 
holding the prey, which is swallowed whole. In 
rachiodon, an African snake, which lives on eggs, the 
teeth are rudimentary, as large teeth would be liable to 
break the eggs, and some or all of the contents might be 
lost : but projecting into the gullet are bony processes 
from the under surface of the vertebrae, which, striking 
against the egg in its passage to the stomach, break the 
shell, and thus save the contents from being lost. Of 
poisonous snakes there are two groups, the colubrine 
and viperine. The cobra is a good example of the former. 
The fang, or poison tooth, is placed on the upper jaw, 
and is always erect; while a few small teeth are placed 
behind it. In the viperine snakes the upper jaw is 
much reduced in size, and carries only one long poison- 
fang. The fang is not always erect, but lies along the 
roof of the mouth, ODly becoming erect when the animal 
opens its mouth to strike its prey. The mechanism by 
which this is effected is thus described by Professor 
Huxley:—“ When the mouth is shut the axis of the quad¬ 
rate bone is inclined downwards and backwards. The 
pterygoid, thrown back as far as it can go, straightens the 
pterygo-palatine joint and causes the axes of the palatine 
and pterygoid bones to coincide. The transverse, also 
carried back by the pterygoid, similarly pulls the posterior 
part of the maxilla and causes its proper palatine face, to 
which the great channeled poison-fangs are attached, to 
look backwards. Hence these fangs lie along the roof of 
the mouth, concealed between folds of mucous membrane. 
But when the animal opens its mouth for the purpose of 
striking its prey, the digastric muscles, pulling up the 
angle of the mandible, at the same time thrust the distal 
end of the quadrate bone forwards. This necessitates 
the pushing forward of the pterygoid, the result of 
which is two-fold : firstly, the bending of the pterygo¬ 
palatine joint; secondly, the partial rotation of the 
maxillary upon its lachrymal joint, the hidden edge of the 
maxillary being thrust downwards and forwards. In 
virtue of this rotation of the maxillary through about a 
quarter of a circle, the dentigerous face of the maxilla 
looks downwards and the fangs are erected into a vertical 
position. The snake ‘ strikes ’ by the simultaneous con¬ 
traction of the crotaphite muscle, part of which extends 
over the poison-gland, the poison is injected into the 
wound through the canal of the fang, and this being with¬ 
drawn, the mouth is shut, all the previous movements re¬ 
versed, and the parts return to their first position.” [The 
mechanism and action of the poison-fang were illustrated 
by an ingenious model, constructed by Mr Stewart.] In 
form the fang is long, pointed, and recurved. Passing 
through the tooth is a canal opening on the anterior surface 
near the jaw and point. This canal is really outside the 
tooth, and might be formed by bending back the edges of 
a flattened tooth till they meet. In the colubrine snake the 
canal is marked by a groove along its anterior surface. 
So large a weapon must run great risk of breakage from 
the struggling of the prey, and to be deprived of it 
