SKELETON 



99 



are comparatively few, the skull is primitive and can only be derived from that 

 of the cotylosaurs. The bones are firmly united, but the sutures are evident 

 (fig. loi). The basioccipital is usually excluded from the foramen magnum, 

 and it and the exoccipitals participate in the tripartite occipital condyle. The 

 supraoccipital is often prolonged into an occipital spine and is fused with the 

 epiotics. The basisphenoid is present, but pre-, ali- and orbitosphenoids are 

 not ossified, a descending plate of the parietal taking the place of the alisphenoid. 

 The pterygoids meet the basisphenoid and may extend to the basioccipital. No 

 ectopterygoid is present. The monimostylic quadrate is large and expanded 

 laterally to support the tympanic membrane, and notched or perforate behind 

 for the columella. 



In the most primitive chelonians a complete false roof is formed by the ex- 

 panded postfrontals, parietals and squamosals. In most of the species the 

 recession of the parietals and squamosals causes a large gap, bounded in front 

 by postfrontal and jugal and exposing the otic bones. Laterally this gap is 



I 



Fig. 



X02. — Hyoid apparatus of Trionyx. b^, b^, first and second branchial arches; 6A, 

 basihyal (copula); A, reduced hyoid; cartilage dotted. 



limited by an arcade of squamosal and quadratojugal, but the latter may be 

 reduced or {Cistudo) absent. In front of the frontals are a pair of bones, which 

 bound the single naris behind. These occupy the position of lacrimals, nasals 

 and prefrontals, and are called by the latter name. The premaxillaries are 

 usually fused; the maxillae have broad palatal processes and trenchant margins. 



'■y, together with the zygomatics, form the lower border of the orbit. 



The vomer is a single vertical plate separating the two choana. The 

 palatines, which bound the choanae behind, are broad and are firmly united to 

 pterygoids and basisphenoid. A parasphenoid is known only in Dermochelys. 

 In the lower jaw the bones are often fused, the two halves being united. Again 

 the bones may be distinct, the splenial being the least constant element. The 

 hyoid apparatus consists of a cartilaginous copula and two pairs of cornua which 

 do not reach the cranium. 



