CHELONII. 



placed on the cartilaginous margin opposite the ends of 

 the ribs ; while in other species of this genus they are entirely 



wanting. 



The case is sometimes covered with a cartilaginous (in the 

 Trionijches) or coriaceous {Sphargis) continuous skin ; but 

 more usually with regular symmetrical horny plates, called 

 sciitella. These shields, when the tortoises are first hatched, 

 are distinct and covered with minute rugosities; they enlarge 

 in size as the animal grows, by the addition of new layers of 

 hcrny matter on their under side, extending beyond their 

 former margin. These additions are generally marked by con- 

 centric grooves, and the rugosity marking the original size of 

 the shield is called the areola. The shields adapt them- 

 selves to the shape of the bones beneath ; and when the bones, 

 as is often the case in the land-tortoises, are raised into 

 conical projections, they assume the same form. They are 

 placed on the bony case in a regular manner, the centre of the 

 back being covered with thirteen plates {sciitella disci) placed 

 in three rows. The central row consists of five shields, which 

 being placed over the vertebrae, are called vertebral plates ; 

 and the side rows of each are designated, for the same reason, 

 from the bones on which they are placed, the costal plates. 

 These are surrounded by 11 or 12 or rarely 13 pairs of 

 smaller plates, called the marginal shields {sciitella margina- 

 lia), added to which there is often a small plate in the centre 

 of the front edge called the nuchal plate, (sciitellum niichale.) 



In all the water tortoises the pair of plates over the tail, 

 called the caudal shields {sciitella caiidalia), are separate from 

 each other. But in the land tortoises, they are united into 

 a single plate, which is often incurved and vaulted. The 

 sternum is covered with six pairs of plates, which for the sake 

 of preventing circumlocution in the description of the species, 

 have been named according to their position ; thus the first 

 pair are called gular {sciitella Qularia) ; the second humeral, 

 {scut, hiivieralia) ; the third pectoral (scut, pectoraliaj ; the 

 fourth abdominal, {scut. ahdominaUa) ; the fifth femoral, 

 {scut.femoralia) ; and the sixth anal, {scut, analia.) Some- 

 times, as in the genus Sternotherus, the gular pair are united 

 into a single plate ; and in others, as in the Chelijdrce and 

 Chelofiice, there is a small plate called the intergular shield, 

 fscuf. ifitergularej placed between them. The symphysis by 

 which the sternum is attached to the upper shell is mostly 

 covered by the outer sides of the abdominal plates (as in the 

 genus Sternotherus) or of the pectoral and abdominal plates ; 



