30 STRUCTURE OF TORTOISES, 



^^ so as to form a great oval plate. Some- 



Fig. 5. jjJE| times this shield is entire, and solid through- 



out its whole extent, sometimes it is divided 

 into three portions, the anterior and posterior 

 of which are slightly moveable, and other 

 times it is hollowed in the centre like a frame ; 

 and, it is fixed on each side to the carapax, 

 either by a broad bony prolongation, or by 

 cartilages, and it is through a sort of slit, or 

 opening left betwixt these two shields or shells, 

 in front and behind this articulation, that the 

 'head, extremities, and tail, are protruded. 



6. The carapax and plastrum are only covered by the ordi- 

 nary skin of this animal which is ordinarily scaly. No muscle 

 is inserted into their surface, and consequently the muscles of the 

 neck and extremities are attached to the interior of the trunk. 

 The shoulder, instead of being sustained by the external parietes 

 of the thorax, is lodged in the interior of this cavity ; and the 

 pelvis, so to speak, is contained in the cavity of the abdomen ; 

 and the marginal pieces of the carapax are advanced beyond the 

 base of the neck and tail, so that these parts seem to project from 

 the same cavities. 



7. The cervical vertebrae, (vc.) far from being immoveable as 

 they are in the back, are so articulated with each other as to 

 permit, in general, of extended movements, similar to those of 

 the neck of a bird. Their number is ordinarily eight, and their 

 articular surfaces are alternately convex and concave, instead of 

 being plain as in mammals. In some tortoises the neck is not 

 retractile ; in others, it can be completely folded beneath the 

 carapax, either by bending downwards on itself, or to one side. 



8. The head of Tortoises is small compared with the body, 

 and the cranium is small compared with the head. Generally, 

 it is flattened, and widened posteriorly by large temporal fossae, 

 which are ordinarily concealed beneath a bony arch. The upper 

 jaw is solidly fixed to the cranium, and is entirely immoveable. 

 The square, or tympanic bone, which supports the lower jaw, is 

 articulated, in the same manner to the adjoining bones, and forms 

 the larger part of the vicinity of the frame of the tympanum ; 

 the lower jaw is composed of a great number of bony pieces : 

 there are six on each side. 



6. Are there any muscular attachments on the external surfaces of the 

 carapax and plastrum ? Is the shoulder on the inside or outside of the chest? 



7. Do the cervical vertebrae move freely on each other or not ? What is 

 their usual number ? 



8. Is the upper jaw of Tortoises moveable ? How is the lower jaw articu- 

 lated with the skull 'f Does the lower jaw consist of a single bone or more ? 



