CHAPTER XIII. 

 SAUROPSIDA. 



THIS great group includes the Reptiles and Birds and 

 forms the second of the three into which the Gnathostomata 

 may be divided. There is nearly always a strongly -developed 

 epiblastic exoskeleton which has the form of scales or feathers, 

 and in some cases a dermal exoskeleton is also well developed. 

 In living forms the notochord never persists, being replaced 

 by vertebrae, but in some extinct forms the centra are 

 notochordal. The vertebral centra are ossified, and only in 

 exceptionally rare cases have terminal epiphyses. The skull 

 is well ossified and has membrane bones incorporated in its 

 walls. 



The occipital segment is completely ossified, and an inter- 

 orbital septum or bony partition separating the two orbits 

 is usually developed to a greater or less extent. The skull 

 generally articulates with the vertebral column by a single 

 occipital condyle into the composition of which the ex- 

 occipitals and basi-occipital enter in varying proportions. 

 The pro-otic ossifies, and either remains distinct from the 

 epi-otic 1 and opisthotic throughout life, or unites with them 

 only after they have fused with the adjacent bones. The 

 hyoid and branchial arches are much reduced ; and the 

 representative of the hyomandibular is connected with the 



1 According to Baur a distinct epi-otic is not recognisable in the 

 reptilian skull. 



