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TORTOISES AND TURTLES. 
shields on the shell, the whole of these tortoises are characterised by the presence of 
an intergular ( i.gu ) shield between the two gulars ( gu ) on the front of the plastron ; 
such intergular shield being, as we have seen, but very rarely present in the 
S-necked group. Very generally among the present assemblage one or more of the 
pairs of costal bones of the carapace may meet in the middle line, owing to the 
absence of some of the median unpaired series of 
bones; in certain cases the whole of the costals 
thus meeting, owing to the absence of all the 
neural bones. Whereas, in one family of the 
group the plastron contains the same nine bones 
as in the side-necked tortoises, in a second family 
there are eleven bony elements in this part of 
the shell, owing to the presence of an additional 
(mesoplastral) pair between the normal hyo- and 
hypo-plastral bones. 
The side-necked tortoises, of which the great 
majority may be included in the two families men¬ 
tioned above, are all of fresh-water habits, and at 
the present day are exclusively restricted to the 
Southern Hemisphere, while they are the only 
members of the order found in Australia and New 
Guinea. During the earlier portion of the Tertiarj^ 
period they extended, however, into the Northern 
Hemisphere, and in the preceding Secondary period 
were abundantly represented in Europe. These 
facts show that the group is a very ancient one; 
and by the presence of the additional mesoplastral 
elements in the lower half of the shell of some 
of its representatives it is allied to a third and totally extinct group, which dis¬ 
appeared before the close of the Secondary period. 
Matamata The extraordinary reptile depicted in the accompanying illustra- 
Tortoise. tion, and known as the matamata (Chelys flvibriata), is the typical 
representative of the first of the existing families of the group— Chelyidce. The 
various genera included therein are collectively characterised by having the normal 
nine bones in the plastron, by the neck being incapable of complete retraction 
within the margins of the shell, and the absence of a bony temporal arch to the 
skull. Eight genera are included in the family, the range of which is restricted to 
South America, Australia, and New Guinea. 
The matamata, which is an American species inhabiting Guiana and Northern 
Brazil, and is the sole representative of its genus, is easily recognised by its broad 
and elongated neck, of which the sides are fringed with peculiar fimbriated pro¬ 
jections, and the depressed and triangular head terminating in a proboscis-like 
nose, and furnished with very small eyes. Not less characteristic is the equally 
depressed and much corrugated shell, in which the carapace bears three longitudinal 
ridges, subdivided into nodose protuberances by cross-valleys; the horny shields of 
the same being extremely rugose, and marked with deep radiating striae. The 
EIGHT HALF OF THE CAKAPACE OF THE 
BLACK STERNOTHERE, WITH THE 
HORNY SHIELDS REMOVED. 
