192 REPTILIA class in 



the Stegocephalian and Rhynchocephalian abdominal ribs. An entoplastron 

 is wanting among the Cinosternidae, and in Baena and the Pelomedusidae the 

 usual number of nine bones is increased to eleven, the additional elements 

 being the mesoplastra, situated between the hyo- and hypo-plastra. The 

 mesoplastra may have served in the primitive condition to close the lateral 

 vacuities. Until late in life in the Chelydridae, and throughout in the Chelo- 

 nidae, the paired abdominal bones are separated by wide fontanelles (Fig. 

 297, 6'), whilst in all other recent Cryptodires the plastron forms in the adult 

 a solid shell, which may be divided into two or three portions by the presence 

 of one or two transverse ligamentous hinges, as in Cistudo, Emys, etc. 



" In the adult of most genera the hyo- and hypo-plastra are united with 

 the marginal plates by suture ; in a few they are narrowly separated from the 

 latter by ligament, or the outer border of these bones form digitate dentations, 

 which may either articulate by gomphosis with the marginals or be entirely free. 

 The space between the body of the plastron and the marginals is called the 

 bridge ; it is particularly short or absent in those Testudinidae in which the 

 plastron is movable, and long and narrow in those forms (Chelydridae) in which 

 the plastron is particularly small, the whole shield being cruciform. In such 

 Testudinidae as have the plastron suturally united with the carapace, the hyo- 

 and hypo-plastron each sends up a process, respectively termed the axillary and 

 inguinal buttress, which anchylose either with the inner surface of the marginals 

 or with the costals ; these buttresses are least developed in the land tortoises 

 and most of the Testudinidae frequenting deep water, in which genera they 

 form very large septa, nearly reaching the vertebral region and forming two 

 lateral chambers occupied by the lungs " (Boulenger). 



Vertebral column. — The cervical region is extremely flexible, and comprises 

 eight non-costiferous vertebrae, the first of which is biconcave, and the last 

 biconvex. Transverse processes are absent or extremely rudimentary in this 

 region. The ten rib-bearing dorsal vertebrae are immovably united with one 

 another and with the carapace, following which are two amphiplatyan sacral 

 vertebrae. The sacral ribs are, as on the last dorsal vertebra, suturallv 

 united with both centrum and neural arch ; the first is the most developed, 

 and considerably expanded distally. The caudal vertebrae range in number 

 between sixteen and thirty-five, the more usual number being from twenty to 

 twenty-five. The centra are in most cases procoelous, but sometimes opistho- 

 coelous. Transverse processes or costoids are present on most of the vertebrae, 

 and connected with the centrum and the arch. Neural spines are not developed, 

 and chevron bones are absent or vestigial. 



Skull. — The bones of the skull form a broad, often very convex roof, which 

 is prolonged posteriorly in a strongly developed supraoccipital crest. The 

 orbits are large and placed laterally in advance of the middle of the skull ; the 

 deeper the skull, the larger the orbits. The latter are completely encircled by 

 four or five bones — the maxilla, prefrontal, sometimes the frontal, the post- 

 frontal, and the jugal. The external nostril is single and terminal, bounded 

 by the premaxillae, maxillae, and prefrontals or nasals. The parietals are of 

 large size and distinct, being connected with the palate (except in the Dermo- 

 chelyidae) by descending processes. An independent lachrymal bone is never 

 present, and nasals occur only rarely among the Pleurodira and some Cryptodira. 

 The anterior margin of the snout is formed by the small, usually distinct 

 premaxillae. 



