THE SKELETON OF THE TURTLE. THE CRANIUM. 225 



of the foramen magnum and is drawn out dorsally into a large 

 crest which extends back far beyond the occipital condyle. 

 In the adult the supra-occipital is completely ankylosed with 

 the epi-otics. 



The Parietal segment. 



The ventral portion of the parietal segment is formed by 

 the basisphenoid (figs. 38 and 39, 4) which lies immediately 

 in front of the basi-occipital. A triangular portion of it is 

 seen in a ventral view of the skull, but it is quickly over- 

 lapped by the pterygoids. It gives off dorsally a pair of 

 short processes which meet the pro-otics. 



The alisphenoidal region is unossified and the only other 

 constituents of the parietal segment are the parietals (fig. 39, 1). 

 These are large bones which, after roofing over the cranial 

 cavity, extend upwards and become expanded into a pair of 

 broad plates which unite with the squamosal and bones of the 

 frontal segment to form a wide, solid, false roof to the skull. 

 Each also sends ventralwards a plate which meets an up- 

 growth from the pterygoid and acts as an alisphenoid. 



The Frontal segment. 



Of the frontal segment the basal or presphenoidal and 

 lateral or orbitosphenoidal portions do not become ossified, 

 the dorsal portion however includes three pairs of membrane 

 bones, the frontals, prefrontals and post/rentals. 



Thefrontals are a pair of small bones lying immediately in 

 front of the parietals, and in front of them are the prefrontals 

 (tigs. 38 and 39, 20), a pair of similar but still smaller bones, 

 which are produced ventrally to meet the vomer and palatines. 

 They form also the dorsal boundary of the anterior nares. 

 The post/rentals (figs. 38 and 39, 18) are larger bones, united 

 dorsally to the frontals and parietals, posteriorly to the squa- 

 mosals, and ventrally to the jugals and quadra tojugals. All 

 three pairs of frontal bones, especially the postfrontals, take 

 part in the bounding of the orbits. 



R. 15 



