GENERAL CHARACTERISTICS. 
45 
as shown in the right-hand figure at the head of the chapter, will be seen to be 
marked by a series of channels corresponding to the borders of these same shields. 
If the shell be not that of a very aged animal, there will be seen in addition a 
number of finely jagged sutures, marking the divisions between the component 
bones, and it will be noticed that in their plan of arrangement, although not in 
number, size, or shape, these underlying bones correspond very closely with the 
overlying horny shields. Thus, in the middle line of the carapace we have a series 
of polygonal plates, symmetrical in themselves, and attached to the summits of the 
vertebrae, which are known as neurcds ; these being clearly indicated in the figure 
referred to. In front, the series is completed by a large nuchal plate, having no 
connection with the backbone, while behind it terminates in one or two pygals, 
■na 
DIAGRAM OF THE HORNY PLATES ON A SHELL OF A FRESH-WATER TORTOISE.— After Gtilltlier. 
which are likewise perfectly distinct from the vertebrae. Externally to the neurals 
are placed on either side the eight costal plates, so named from being attached to 
the ribs; the inner halves of these plates being alone visible in the shell figured at 
the head of the chapter, which belonged to a rather aged animal. Finally, the 
edges of the carapace are formed by the marginal plates, which, like the horny 
shields similarly named, are angulated, and form the lateral borders of the middle 
portion of the plastron. In the plastron itself, we find its anterior portion formed 
by a pair of plates, known as the epiplastrals, corresponding to the collar-bones, or 
clavicles, of other Vertebrates; while between or behind these is a single unpaired 
entoplastral element, which may be either dagger-shaped or rhomboidal, and which 
represents the interclavicle of less specially modified reptiles. The remainder of 
the plastron is formed by three pairs of plates, respectively known as the hyo, hypo, 
and xiphiplastrals, of which the latter or hindmost are generally more or less 
deeply notched or forked. These three elements appear to correspond to the so- 
called abdominal ribs of crocodiles ; and it will thus be evident that Chelonians have 
