Fig. 66.–The bones composing the plastron of Chelone mydas. On the right side the position of the covering horny shields[[128]] is indicated by dotted lines. a, Anal horny shield; ab, abdominal; f, femoral; g, gular; h, humeral; ig, intergular; im, infra-marginals; p, pectoral.
The shell, which is the most characteristic feature of the Chelonia, consists of the dorsal "carapace" and the ventral "plastron." Each is composed of a considerable number of bony plates which arise as ossifications of nearly the whole thickness of the cutis, only a thin layer of subcutaneous connective tissue remaining soft and lining the inside of the shell. We restrict ourselves to a description of the shell of the Thecophora, leaving the discussion of the peculiar shell of Sphargis to p. [336] f. Very young tortoises are still soft, and the plates which are beginning to ossify are not yet suturally united. The plastron (Figs. 66 and 67) consists of the paired epi-, hyo-, hypo-, and xiphi-plastral plates, and the unpaired endo-plastral plate.
Fig. 67.–Bony shell of Testudo ibera. A, Ventral; B, dorsal; C, left-side view. In B, and on the right half of A, the position of the horny shields is indicated by dotted lines. The underlying bony plates are marked by strong lines. In B the 1st neural and costal plates, the 4th neural, costal, and 6th marginal plates, and the 7th neural plate are shaded. 1, 4, 6, First, fourth, and sixth neural plate; M, in C, fifth left marginal plate; Nu, nuchal plate.
The latter is homologous with the interclavicle, the epi-plastra are homologous with the clavicles of other Reptiles, while the other pieces are genetically derived from, and are further modifications of, the so-called abdominal ribs of the Crocodilia and Prosauria, These plastral plates are never in direct contact with the shoulder-girdle or with any other parts of the internal skeleton. In the young of all tortoises, and in the adult of the Chelonidae and Trionychidae, the several plastral plates enclose large, irregularly-shaped fontanelles. These are more or less filled up in the other groups; and in the Testudinidae especially the whole plastron forms one continuous mass. The navel is situated between the hyo- and hypo-plastrals. Both these pairs are broader than the others, and are connected with the carapace by means of several marginals. The connecting region is called the bridge. In several tortoises, e.g. Emys, the connexion with the marginals is formed by ligaments only and remains movable. In others, transverse, more or less perfect hinges are formed across the plastron. A rather imperfect joint between the hypo- and xiphi-plastrals develops with age in Testudo ibera. In Cistudo and Cyclemys a very effective hinge lies below the hyo- and hypo-plastrals, just in front of the bridge; and the anterior and posterior lobes of the plastron can be closed against the inner rim of the box, fitting tightly in Cistudo. In Pyxis the front lobe only is movable.
Fig. 68.–A, Diagrammatic transverse section through the shell of Testudo. On the right side the horny shields have been removed, on the left are shown the neural, costal, marginal, and pectoral shields. The bony dermal plates are dotted. Cap, Capitular portion of rib; Sp.C, position of spinal cord. B, Vertical section through part of the shell, magnified and diagrammatic. B, Bony layer of the cutis; L, leathery layer of the cutis; M, cells of the Malpighian layer; P, star-shaped pigment-cells; SC, stratum corneum, composing the horny shields.
The carapace is composed of one median series, a right and left lateral series of costal plates, and a series of marginals which surround the whole. The median series consists of one large nuchal plate, normally eight neurals and one to three supracaudal plates. The characteristic feature of the neural plates is that they are firmly fused with the broadened neural spinous processes of the underlying vertebrae. The nuchal plate lies in front of the first thoracic or ninth vertebra; it overlies the last cervical vertebrae, with the eighth of which it is connected by ligament only; but the posterior corner of the plate often fuses with the spine of the ninth vertebra. In the Chelydridae, and still more in the Trionychidae, the nuchal sends out a pair of long rib-like processes, which either extend to below some of the neighbouring marginals, or their ends overlap those of the ribs of the second thoracic vertebra (e.g. Trionyx), or, lastly, they are in turn overlapped by the first costal plates (e.g. Cyclanorbis). Such rib-like processes are also present, well developed in the young, shorter in the adult, in the Dermatemydidae and Cinosternidae. It is possible that the nuchal plate represents the fused neural of the eighth and the costal plates of the ninth vertebrae. An indication of the compound nature of the nuchal may be found in the fact that two nuchals have been described in Chelydropsis carinata, a Miocene relation of Chelydra. Somewhat similar modifications have taken place in the post-sacral region. The one to three supracaudal plates are, namely, neurals which have lost their connexion with, or perhaps have never been fused with, the spinous processes of the movable tail-vertebrae. The number of neural plates is mostly eight, but there are sometimes individually nine or ten, the gradual suppression taking place first in the sacral region. When such a plate is suppressed the neighbouring costal plates usually close up and meet in the median line. In Cistudo, for instance, there are only seven normal neurals, the eighth pair of costals meet, and the original eighth neural is transformed into a supracaudal. In Cinosternum the sixth to eighth costals meet, separating the one supracaudal widely from the remaining five neurals. The meeting of the last pair of costals, with co-ordinate reduction of the neurals to seven, is almost universal in the Pleurodira; and this tendency is carried out to an extreme in the Brazilian Platemys and in the Australian Chelodina and its allies, in which all the costals meet in the middle line, and the neurals are completely suppressed. Every stage intermediate between complete neurals (Sternothaerus) and interrupted, vestigial, and vanished neurals, is still represented by some genus. This process takes place independently, both in America and in Australia, and is one of the most recently introduced modifications.
The costal plates arise, like the neurals, independently in the cutis, but they soon come into contact with the underlying cartilage of the ribs, which are long enough to reach the marginals. The ribs flatten, become surrounded by the growing membrane-bone of the plates, and the cartilage of the ribs, instead of ossifying, undergoes a process of calcification. Ultimately this is more or less absorbed, its place is taken by the dermal bone, which forms so to speak a cast of the rib, preserving in many cases the shape of the vanished rib, only the capitular portions of which remain unaffected. The number of costal plates is very constant, namely eight on each side, but some fossils have nine or ten, and there are still individual variations in recent forms, indicative of that number. In a large Chrysemys concinna I find the last pair of costals clearly composed of at least two pairs, and this same specimen has nine distinct neural plates.