136 VERTEBRATE SKELETON 



sometimes in the adult (as in Chelonia) the exoccipitals may con- 

 tribute to it; 3, the existence, except in snakes and some turtles, of a 

 high (cartilage or osseous) interorbital septum, continued forwards 

 as a nasal septum; 4, as a result, the cranial cavity is restricted to the 

 postorbital cavity and the presphenoid is reduced; 5, the large 

 number of distinct bones appearing in development, especially in 

 the orbital and temporal regions and the frequent occurrence of 

 sclerotic bones. 



There are two hues of cranial structure in Sauropsida, strepto- 

 stylic and monimostylic. The former includes only Squamata and 

 birds; the latter all other orders, living and extinct. In the latter 

 the quadrate is held firmly by the squamosal and other bones; in 

 the streptostyhc group the quadrate moves freely on the cranium, 

 this mobility often extending to the pterygoids (which do not meet 

 in the median line) and occasionally (some birds) to the anterior part 

 of the cranium. The number of bones appearing in development is 

 greater than in mammals or existing Amphibia, but the number is 

 often reduced in the adult by fusion, especially in birds. 



REPTILIA. — The skull of most reptiles differs from that of 

 birds in the greater number of bones in both young and adult. The 

 relations of the foramen magnum are more primitive in reptiles in 

 being directed backwards instead of downwards. Pterosaurs forming 

 an exception to this general statement. As a result of the smaller 

 brain, the reptilian otic capsules form a part of the lateral cranial 

 wall, instead of its floor, while the squamosal, in all lower groups is 

 excluded from the wall of the cranial cavity. 



As in all Amniotes, the chondrocranium (fig. 144) is distinctly 

 tropibasic. The trabecula communis passes dorsally into inter- 

 orbital and nasal septa, and in front to structures comparable to the 

 trabecular cornua and ethmoid plate of Ichthyopsida. The inter- 

 orbital septum separates dorsally as a pair of supraseptal plates 

 (fig. 68) which he immediately below the olfactory nerves. The 

 basal plate may be entire or may have one or more basicranial fenes- 

 trae, the lagenar part of the ear extending into it. The chondro- 

 cranial roof is poorly developed, there being a narrow synotic tectum 

 sometimes with a narrow bar extending forwards from it. 



The preotic parts are relatively lighter, more slender and more 

 complicated than in Ichthyopsida, the lateral walls in front of the 

 otic capsules being reduced to one or more bars (an upper marginal 



