448 THE VERTEBRATE SKELETON. 



are very much developed, the postzygapophyses being semi- 

 cylindrical and fitting into the deep prezygapophyses of the 

 succeeding vertebra. 



In the SIRENIA the number of lumbar vertebrae is very 

 small ; in the dugong there are nineteen thoracic and four 

 lumbar, and in the manatee seventeen thoracic arid two lumbar. 



In the CETACEA the number of thoracic vertebrae varies from 

 nine in Hyperoodon to fifteen or sixteen in Balaenoptera, and 

 the number of lumbar vertebrae from three in Inia to twenty- 

 four or more in Delphinus. The lumbar vertebrae are often 

 very loosely articulated together and the zygapophyses some- 

 times as in the Dolphins are placed high up on the neural 

 spines. The centra are large, short in the anterior region but 

 becoming longer behind. The epiphyses are prominent, and 

 so are the neural spines and to a less extent the metapophyses. 

 The transverse processes are well developed, anteriorly they 

 arise high up on the neural arch, but when the vertebral 

 column is followed back they come gradually to be placed lower 

 down, till in the lumbar region they project from the middle of 

 the centra. This can be well traced in the Porpoise (Phocaena). 

 In the Physeteridae the transverse processes of the anterior 

 thoracic vertebrae are similar to those of most Cetacea, but 

 when followed back, instead of shifting their position on the 

 vertebrae, they gradually disappear, and other processes 

 gradually arise from the point where the capitulum of the rib 

 articulates. 



UNGULATA. In the Ungulata vera the thoraco-lumbar 

 vertebrae are slightly opisthocoelous. The anterior thoracic 

 vertebrae commonly have exceedingly high backwardly-pro- 

 jecting neural spines (fig. 89,1) ; but those of the lumbar and 

 posterior thoracic vertebrae often point somewhat forwards 

 so that the spines all converge somewhat to a point called the 

 centre of motion (cp. fig. 101). In the Artiodactyla there are 

 always nineteen thoraco-lumbar vertebrae, and in the Perisso- 

 dactyla twenty-three. 



