NATURAL HISTORY. 
107 
ROOM II.] 
at the same time, twisting the end of the tail round a tree, or some 
other fixed point, in order to increase their power. The American 
kinds {Boa) have only a single row of plates beneath the tail; they 
vary greatly in respect to the structure of the scales on the head and 
lips. Some of the Indian species, as the Netted Boa, ( Boa Regia,) 
have a single series of plates, whilst most of the other species have two 
rows {Python). The Pythons cover their eggs by laying on them 
rolled up in a spiral form, with the head in the centre. The Eryx differs 
from the other Boas in having a cylindrical body, a very short tail, and 
the head covered with scales similar to those of the back. Cuvier says 
that this genus has no spurs, but the specimen in the British Museum 
shews them distinctly. 
The family of Sea-Snakes (Hydride, Case 19) are easily known by 
their compressed form, narrow scale-like ventral shields, and vertically 
flattened tail. Their hinder limbs are not developed. The eyes and 
nostrils are vertical and the pupil round. These reptiles, which are 
peculiar to the seas of Asia and New Holland, are in some degree poi¬ 
sonous, many of the species having small fangs, dispersed amongst the 
true teeth. Some have a small head, and the body covered with scales, 
( Hydrus); the others have a large head and broad neck, and the body 
covered with embedded square plates, placed in longitudinal series, as 
Pelamis. The Achrochordus has the habits and many of the characters 
of the Sea-Snakes; but its body and head are covered with rough 
granular scales, and its tail is conical. It is found in the rice-fields of In¬ 
dia. The Chersydy'us has the scales of the Achrochordus but the tail 
is compressed, as in the other Sea- Snakes. The Homalopses have the 
narrow 7 belly plates, the form and scales of the Boce, but they are des¬ 
titute of spurs, and have the nostrils and marine habits of the Sea- Snakes; 
and they are generally slate-coloured. Some of the Sea-snakes grow 
to a very large size, they sleep on the shores curled up, and are some¬ 
times found asleep on the surface of the calm tropical seas. 
II. The shielded Reptiles, Cataphracta, (Cases 20 to 24) have 
the body covered with square imbedded shields, and have the tympanic 
bones sunk in the base of the solid hard bony skull; the vent is round¬ 
ish, or linear and plaited, and the generative organs are usually simple. 
They comprise the Tortoises, ( Chelonians ,) Emydosauri, and Amphis- 
benians. 
The Tortoises (Chelonians, Cases 20 to 33)are peculiar for having 
all the muscles of the body placed within the thorax, out of the front and 
hinder part of which the head, limbs, and tail are exserted, as out of a 
case. This case is formed of the dilated back bone, ribs, and breast 
bones. When the animal is first born, these bones are separate from 
each other, as in the other Vertebrata, but as they grow, they gradually 
enlarge, and approximate, and at length are united together by a toothed 
suture, until they form a very substantial protection to the body. They 
are divided into families, according to the form of their feet, w hich vary 
w ith the habits of the animals. 
Those which live on land ( Testudinhle, Case 20) have club-shaped 
feet, and very solid, convex shells, with a single marginal caudal plate. 
These live chiefly on vegetable substances, and bury themselves in the 
ground during the -winter: their eggs, some of which are exhibited, are 
of a globular form. These animals are often used as food, especially 
