46 VERTEBRATE SKELETON 



to four sacrals, while there may be forty or more caudals. The neural arches 

 often are not fused with the centra and the haemal arches of the tail are not 

 complete, the haemapophyses not meeting below. The short cervical ribs are 

 articulated to the centrum only, while the trunk ribs are single or double headed, 

 the former more common in the older groups. 



ICHTHYOSAURIA have numerous (120-150) very short vertebrae, divided into 

 caudal and precaudal regions, there being no sacral region. The precaudals, 

 atlas and epistropheus excepted, bear bicipital ribs which articulate with di- and 

 parapophyses, both on the centrum. The caudal ribs have single heads. Atlas 

 and epistropheus are almost always fused, and in front of and between them are 

 wedge-shaped intercentra. The neurapophyses of the atlas do not meet above 

 and the haemapophyses in the tail also fail to meet below. A peculiarity of the 

 tail is a strong deflection of the vertebral column, the axis entering the lower 

 lobe of the caudal fin, thus reversing the conditions of the heterocercal tail of 

 fishes. 



Some DiNOSAURiA have amphicoele, some opisthocoele and some amphi- 

 platyan vertebra;, and different kinds of centra may occur in the same column. 

 All five regions are differentiated, both cervical and caudal sometimes being very 

 long. Atlas, epistropheus and one or two anterior cervicals are fused in some 

 Sauropods and there is no suture between cervical ribs and vertebrae. The 

 thoracic ribs are very long, sometimes with vertebral, intermediate and sternal 

 portions. The sacrum includes from two to six fused vertebrae and some 

 stegosaurs had a synsacrum, vertebrae being added from lumbars and caudals, 

 sometimes to the number of ten. This synsacrum is correlated with the great 

 size of the body, the small fore hmbs and the bipedal locomotion. Also, in 

 correlation, is the great size of the vertebral canal in the sacrum, indicating that 

 the spinal cord here was larger than the brain. Another correlation is the 

 fusion of the spines of the sacral vertebrae and the connexion of the neural 

 arches with the iliac bones, dorsal to the sacral ribs. These sacral ribs were 

 often separate, and in Ceratosaurus they alternated with their centra. Many 

 Dinosaurs have several caudal ribs and in Diplodociis the chevrons are double, 

 each with anterior and posterior branches. In some the longitudinal ligaments 

 (p. 19) were ossified. 



Pterosauria have procoelous presacrals, the caudals being amphicoelous. 

 There were seven cervicals, twelve to sixteen dorsals and from four to seven 

 synsacrals. The cervical ribs (sometimes absent), are bicipital; other ribs have 

 one head and are sometimes coossified with the centra as are the neural 

 arches. Sacrals and synsacrals are usually fused; chevron bones occasionally 

 occur in the tail. 



AVES. — In birds cervical, thoracic, sacral and caudal regions are 

 always present, but the lumbars are absorbed in the synsacrum to a 

 greater or less extent. ArcJicEopteryx and Ichthyornis have slightly 

 amphicoele centra as do the embryos of existing birds, the caudals of 

 other birds, and the thoracic vertebral of penguins, plovers, auks, 



