228 VERTEBRATE ZOOLOGY 



tips of the spinal processes of the vertebrae and of the much flattened 

 ribs; there are usually eight neural plates and eight pairs of costal 

 plates. In front of the first neural is a dermal plate, the nuchal; 

 back of the eighth neural are usually three dermal plates, the first 

 and second procaudals and the pygal. Around the margin of the 

 carapace are usually eleven pairs of dermal plates, the marginals. 

 Overlying the bony carapace there is a horny carapace composed of 

 five neural scutes, four pairs of costals, a small anteriorly placed nuchal, 

 and twelve pairs of marginals. This elaborate composition prevails 

 in nearly all of our modern turtles as well as in many species long 

 extinct. 



The plastron (Fig. 129, B), like the carapace, is composed of two 

 kinds of bony elements covered with horny elements. The bony 

 elements consist of four pairs of plates: the epi-, hyo-, hypo- and 

 xiphi-plastrals. The epiplastrals are the modified clavicles, the 

 hypoplastrals and xiphiplastrals are broadened abdominal ribs, the 

 hyoplaslrals appear to be dermal elements without homologies. A 

 small median dermal element between the epiplastrals and hyoplas- 

 trals is called the endoplastrals. There are usually six pairs of horny 

 scutes that break the joints of the bony plastron. The pillars be- 

 tween the carapace and plastron are derived from the hyoplastrals 

 and hypoplastrals. 



The conventionalized pattern of bones and scutes in the armature 

 has evidently been arrived at after a long period of evolution. Many 

 evidences indicate that the ancestral condition was much more plas- 

 tic and variable and that there were originally many more plates and 

 scutes than at present. By dropping out both longitudinal and trans- 

 verse rows of elements the whole system has been greatly simplified. 

 Most species of turtles to-day show a certain percentage of individuals 

 with supernumerary scutes and plates, that are evidently vestiges of 

 ancestral conditions. 



The vertebrce in the trunk region are rigidly united to the narrowed 

 bases of the paddle-like ribs. They are not very numerous: 8 cervi- 

 cal, 10 thoracic, 2 sacral, and a variable number of caudal vertebrae, 

 which are proccelous in form. 



One of the most puzzling features of the skeleton (Fig. 130) is the 

 peculiar position of the limb girdles. Both pectoral and pelvic gir- 

 dles are inside instead of outside of the ribs. How they got inside is 

 a mystery that not even a study of their embryogenesis is able to 



