The lower jaw of the tortoises is divided in a manner which it is not very easy to refer to that manifested in the crocodile, to which, Curier observes, that of the birds has a much more striking relation ; but the bird's jaw, he adds, also approaching to that of the tortoises, aide us in referring it to a common type. The space occupied in the crocodile by the two dental and the two opercular bones is filled in the Marine Tortoises, the Fresh-Water, and Land-Tortoises, as well as in the Trionycca, with a single bone only, the analogue of the two dental bones. Cuvier never saw in all these sub-genera, even in their youth, any trace of eymphyeis: the bone is continuous in the tortoises, as in birds. The Matamata, or Chdys, on the contrary, preserves in every age a division at the anterior part. The opercular bone always exists, as in the crocodile, at the internal surface ; but it is carried farther backwards, and attains to the posterior extremity. Beneath it is the angular bone forming the lower edge of the jaw. That which Cuvier names the surangular bone occupies the external surface of this part of the jaw, and proceeds also to its posterior extremity, but only touches the angular bone quite behind, and in becoming separated on the two anterior thirds by a long point of the dental bone. Above, and towards the back part, between the opercular and surangular bones, the articular bone is situated, as in the birds ; but in the tortoises it is reduced to smaller dimensions, only serving for the articulation and for the insertion of the depressor much), or the analogue of the digastric inurich). The coronoid apophysis does not belong at all to the surangular bone in this order, but to a bone placed between the dental, the opereular, and tho surangular bones; and in front of the aperture by which the nerves enter the jaw, an opening, which is here found at the upper border, instead of being, as in the crocodile and the birds, at the internal surface. This bone, which is not found in the birds, can only respond to the complementary bona in the crocodile. Cuvier saw in the Emys expansatho surangular, the opercular, and the articular bones anchylosed, and their sutures effaced, at a period when all the others were still visible. The general form of the bony jaw corresponds nearly to what is seen externally. More pointed in the Trionym and Cltelone Cs.rshus ; more obtuse, more parabolic, in CZ 21/ydas and the Land-Tortoises ; semi circular in front of the coronoid apophyses in the 3latamata; it differs also in the furrow with which it is hollowed. This furrow is narrow, deep, and equally wide in the Land-Tortoises ; widens and deepens towards the syrnphysis in C. 91 ; and is entirely wanting in Trionys, C. Careita, &e.
The os hyohlee of the tortoises is more complicated than that of the crocodiles, and varies singularly in form from one genus and even one species to another. It is in general composed of a body itself, some times subdivided into many pieces, and of two, sometimes three pairs of horns : and under the anterior pert of its body is, besides, suspended a bone or a cartilage, sometimes double, which is the true bone of the tongue analogous to that seen in the birds, but articulated in them in front of the body of the os hyoidea, whilst in the tortoises it is sus pended below it The greatest horns (the anterior pair when there are only two, the middle when there are three, representing the styloitlean bones) embrace tho cesophague, and mount behind the muscles which aro the analogues of the digastrics, or depressors of the lower jaw, but without being fixed otherwise than by their proper muscles. The Land-Tortoises have the body of the os hyoides wider, its anterior portion longer, and want the small anterior horns, whilst the anterior angle is very much developed. In the middle of the disc
are two round spaces, which in certain tortoises, the Test udo Indica for example, are only more delicate ; but which in the others, Testudo radiate for instance, are absolutely membranous.
In some Fresh-Water Tortoises, Tuatudo Enroprea and T. clause for example, the body of the bone is longer than it is wide ; and has in the front a small membranous space, and at its anterior angles the email lateral Lorna Sometimes two or even four osseous nuclei arc there formed.
The os hyoides of Trionys differs still more. Its body is composed in front of a cartilaginous point, under which is suspended a great lingual oval cartilage. At the base of this a rhomboidal osseous piece adheres on each aide, which piece represents the anterior horns, and afterwards four others forming a thick disc, cone/I've above, wider in front, and notched on the sides and behind. At the anterior angles of this disc adhere the middle horns, and to the posterior angles are attached the posterior horns : all four are very bony. The middle are formed by a long piece, which is compressed, arched, and terminated by a small cartilage. The others aro wider, flatter, and prolonged by a cartilage, in the substance of which aro encrusted in a row from five to six bony nuclei, which are round or oval, very hard and very distinct ; so that the entire bone comprehends twenty different osseous pieces, which appear to remain distinct to old age.
The most singular of all these is that of the Chelp, and is very early entirely ossified. Its body is composed of a long narrow pris matic piece, hollowed above by a canal where the trachea runs. In front this piece is dilated, and carries on each side two angular por tions, four in all, without counting the piece itself. The two interme diate ones unite in front, leaving between them and the principal body a membranous space on which the larynx reposes. The lateral por tions, Cuvier observes, represent perhaps the small anterior horns. It is on the angle which they form with the dilatation of the principal body that the middle horns are articulated ; these last are very strong, prismatic on their internal moiety, and then slender, and ter minated by a bony and pointed piece, distinct from the rest of tho horn. Tho posterior horns are articulated at the posterior extremity of the prism formed by the principal body. They are long, strong, slightly compressed, and curved into an arch.
Under the anterior and dilated part is suspended the true bone of the tongue, formed in front of a semicircular cartilage, and behind of two bony pieces in form of a crescent, the internal angle of which is prolonged into a sort of tail or pedicle, which lies under the prismatic body of the os hyoidea.
In the Turtles, Clalonc Caretta for instance, the body of the bone is in the form of an oblong buckler, concave above for the support of the larynx and the commencement of the trachea, and drawn out in front into a point which penetrates into the flesh of the tongue in passing upon the lingual bone. It presents on each aide an angle for carrying the anterior horn, which is very small ; the great horn enrved into an obtuse angle for going round the cesopliague and jaw, more bony than all the rest of the apparatus, is articulated to the middle of the lateral border of the body of the bone, and its free or upper extre mity is terminated by a small cartilaginous articulation. The posterior horns are articulated to the posterior angles. They are cartilaginous, flat, rather wide, and scarcely arched.