ii5° 



CHAPTER LV. 



CLASS REPTILIA — continued. 



Orders Dinosauria, Crocodilia, and Ornithosauria. 



Archosaurian Branch. — The three orders constituting this 

 branch comprise the most highly developed of all Reptiles, and 

 those which make the nearest approach in their organisation to 

 the Avian type. They also include the largest forms yet known, 

 not only among Reptiles, but also among all Vertebrates adapted 

 for a life on land. The following features are common to the 

 entire branch. The teeth are very generally implanted in distinct 

 sockets ; are never anchylosed to the bones of the jaws ; and are 

 exclusively confined to the premaxilla, maxilla, and dentary bones. 

 Both the pectoral and pelvic limbs are always well developed ; the 

 cranium has no parietal foramen, but is furnished with both a 

 superior and an inferior temporal arcade (as is shown in the figure 

 of the Crocodilian skull on p. 1181); the quadrate is firmly fixed 

 among the adjacent bones ; and there is frequently no columella. 

 The anterior ribs have a distinct capitulum and tuberculum ; the 

 dorsal vertebrae carry long transverse processes, which may be 

 placed entirely on the arch ; and there may be more than two ver- 

 tebrae in the sacrum. There is never a T-shaped interclavicle, and 

 the only indication of the precoracoid is afforded by the fontanelle 

 in the coracoid, which indicates its original duality. The humerus 

 has no foramen, but an ectepicondylar groove may occur. The 

 proximal row of the tarsus comprises two bones, representing the 

 astragalus and calcaneum. Abdominal ribs are generally present. 

 The number of phalangeals in the limbs approximates more or less 

 closely to that obtaining in the Streptostylic branch, although in 

 some cases there is a reduction. The lateral surface of the mandi- 

 bular ramus may have a vacuity. As a rule the centrum of the 

 atlas vertebra forms an odontoid process more or less closely at- 

 tached to the centrum of the axis ; the arch of the former being 



