CRYPTODIRA, 13 
the median line. In a specimen of Cinosternum albogulure, the first 
neural plate is also absent. The shape of the neural plates affords 
good generic characters. They may be tetra-, hexa-, or octagonal ; 
they may be all similar in shape (hexagonal), or hexagonal plates 
may alternate with octagonal, or tetragonal with octagonal (e. g. 
Testudo). When hexagonal, their lateral sides may be of equal or 
subequal length (e.g. some Chelonidie), or the antero-lateral (c. g 
Emys) or the postero-lateral (e. g. Cyclemys) the shorter ; the hinder- 
most plates are as a general rule short-sided in front; the first is, 
with few exceptions, tetragonal. It not unfrequently happens for 
one or more of the neural plates to split up into two or three, or for 
two to amalgamate into one, but such are usually easily recognized 
as anomalies. 
The costal plates number eight on each side*. In the newly- 
hatched young they are scarcely developed, and the ribs do not come 
into contact with one another; ossification starts a short distance 
from the neural plate and soon works up towards the latter ; but it 
takes some considerable time until it reaches the distal extremity, a 
stage which is attained but late in life in Chelydra and Macro- 
clemmys, and very late or perhaps never in the marine Turtles. In 
Batagur small fontanelles persist between the marginals and costals 
in specimens which haye nearly attained full size. As a rule, the 
carapace of land Tortoises ossifies more rapidly than that of fresh- 
water. The distal extremity of the rib persists as a free point 
fitting into a corresponding socket in the marginal plate. The first 
costal plate is constantly the broadest (per pendicularly to the axis), 
the last the smallest ; in Cinosternwm the latter is unusually small 
and destitute of the distal point. In Vestudo, Homopus, and Ciniys, 
and to a lesser degree in a few freshwater Tortoises, especially old 
individuals, these plates are unequal in width, alternately widening 
proximally and narrowing distally. 
Continuing the series of the neural plates are two to four azygous 
membrane bones termed the pygal plates, the last of which, in shape 
and position, forms part of the marginal series; this distinction is 
most obvious in immature Chelonidee, the azygous marginal being 
separated from the true pygals by an interspace. 
In Thalassochelys usually and in Chelone exceptionally, the last 
pair of costals meets on the middle line, between the last neural and 
the anterior pygal, of which two or three are present besides the 
azygous marginal. 
Tn addition to the azygous posterior marginal, there are eleven 
(exceptionally twelve) marginal plates on tech side, except in 
Staurotypus and Cinosternum, which have only ten. 
Puastron.—The plastron is composed of nine bones 7, viz. an 
* Nine or ten in some fossil marine forms. 
+ In the fossil Baénidee an additional bone (mesoplastron) is present on each 
side, between the hyo- and the hypoplastron. 
