The tenth rib, attached between the bodies of the tenth and cleveuth vertebra:, produces no plate and enters not into the com position of the dorsal buckler. Like the first, it has only a portion of the head, and is joined by its other extremity to the internal surface of the ninth.
The eleventh vertebra after the cervical is the only one that can be termed lumbar; it carries no rib. In the Turtles, its annular portion again gives a plate to the longitudinal series of the dorsal buckler, and is the tenth and tho smallest of the pieces of this series. The twelfth and thirteenth vertebra aro the eacmL At their sides are attached two lateral pieces similar to the beads of the ribs, but stronger, especially the first, and convex at the end, in order to their union with the posterior and upper angle of the °gm iii. Their annular portion is close and complete, and is not incorporated with the plates of the buckler which follow that of the eleventh vertebra. The vertebrae of the tail are free, like those of the neck : hence the plates of the longitudinal series, which follow the tenth, do not adhere to the vertebrae, and, if they belong thereto, only so belong by a metaphysical relation, and accordingly they may be considered as having been dismembered. So of the first of all the plates of the series. It only furnishes an attachment to the annular portion by synchondrosis, otherwise close and complete, of the first dorsal vertebra ; and if one would regard it as belonging thereto, it would be necessary to consider it as dismembered.
The Turtles have three longitudinal plates after the tenth, making thirteen in all ; but the second is sometimes divided into two, and the ninth also, which increases their number to fifteen.
Cuvier found fourteen in some of the Emydes, the Emys serrata for instance ; but the eleventh and twelfth, he adds, are very small in them. There is but a single one after the tenth in the Land-Tortoises and the Chelydes, so that they have only eleven in all. It sometimes happens that one or two of these plates are not seen externally. Thus in the Box-Tortoises, the two ribs of the last pair are joined to each other, and thus cover the ninth plate ; and in this respect many modifications occur in the same species; • of which Bojanue has, in his third plate, given many examples taken from the European Tortoise.
In Chelys the last and penultimate rib are attached to the eighth plate, and the ninth remains hidden. In both cases the tenth and the eleventh subsist as ordinarily.
In the Turtles, the eight pairs of ribs and the thirteen plates of • the longitudinal series form a slightly convex oval buckler, a little narrowed backwards. The ribs are not incorporated throughout their length, a narrow fraction remains towards their exterior, and the intervals between this portion and that of the anterior and posterior ribs are filled up by a cartilaginous membrane only. It is only in extreme old age that some are widened to the end. Cuvier had sometimes seen the first three and a part of the fourth in this state.
In the Fresh-Water Tortoises and in Chelys the buckler is entirely filled up in time, and the ribs incorporate themselves throughout their length, between each other and with the marginal pieces. The ossification pro ceeds still faster in the Land Tortoises, and it is only in their youth that vacant spaces are observed between the ex ternal parts of their ribs.
The Sternum Plastron, or Breast-Plate is always com posed of nine pieces, of which eight are pairs, and the ninth is odd and always placed be tween the four anterior ones, with the first two of which it generally coheres, when it is not articulated with the four.
These nine pieces vary much in figure according to the genera and species.
In the Land and Fresh-Water Tortoises and in Che/ys they only leave between each other in early youth, when they are formed by bony rays shooting in various directions in the still cartilaginous disc of the plastron, like the bones of the cranium in the foetus of mammals ; but, with age, these rays join each other from every side, and form a disc compact in all its parts, which unites itself by a more or less considerable extent on each side to the dorsal buckler.