SKELETON. gI 
plesiosaurs in the angle between palatine and transversum. The usual bones are 
frequently distinct in the lower jaw. ; 
In the CHELonzans the cranial cavity extends forward between the eyes and the 
mesethmoidal cartilage largely persists in the adult. Although the bones 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. The 
basioccipital is usually excluded from the foramen magnum, and it and the ex- 
occipitals participate in the tripartite occipital condyle. The supraoccipital is 
often prolonged into an occipital spine and is fused with the epiotics. The basi- 
sphenoid 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 
Fic. 93.—Hyoid apparatus of Tryonyx. 51, b?, first and second branchial arches; bh, 
basihyal (copula); , reduced hyoid; cartilage dotted. 
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 expanded 
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 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 premazxillaries are usually fused; the maxilla have broad 
palatal processes and trenchant margins. They, together with the zygomatics, 
form the lower border of the orbit. 
The vomer is a single vertical plate separating the two choanez. ‘The palatines, 
which bound the choanz behind are broad and are firmly united to pterygoids 
and basisphenoid. A parasphenoid is known only in Dermochelys. In the lower 
