The British Guiana Museum. 201 
gills instead of being, as in ordinary fishes, movable, 
closely gathered and covered by a movable flap loose 
behind to allow of the escape of the water taken in at 
the mouth for respiration, are fixed in pouches which open 
separately to the exterior on each side behind the head, 
being usually five, though occasionally six or seven in 
number. The elongated body, the well-developed snout 
with the mouth placed well on the under side, and the 
strong tail forming a prolonged upper lobe, are other 
special features of the sharks. These sea-monsters have 
been obtained more than thirty feet in length. 
In front are seen two colonial Tortoises. The smaller 
specimen is a very large form of the common Land 
Tortoise ; while the larger specimen is a River Tortoise 
{Podocnemz's expansa), which had evidently reached a 
good age, evidenced by the deep concavity along the 
middle of the back — in the young state, the back is 
markedly convex. The limbs of the River Tortoises, 
though modified for swimming, are not converted into 
perfect fins, and they still retain distinctly visible 
webbed digits (with long nails) which are not notice- 
able in the perfect fins of the marine forms or Tur- 
tles The toothless, horny, beaklike jaws of this order 
(Chelonia) are peculiar among the class of Reptiles, as 
is also the hard bony box, enclosing the body, and made 
up of plates developed in the skin and fused with the 
expanded ribs and backbone of the true skeleton within. 
The Tortoise-shell of commerce is derived from the 
outer plates of this box in the Hawk's-bill Turtle. 
Existing Tortoises are but pigmies when compared with 
extinft forms, one of which, discovered in India, appa- 
rently attained a length of about twenty feet. 
