558 CLASS XVII. 



eigliteen, the rhinoceroses have nineteen or twenty, the elephants 

 from nineteen to one and twenty, and the three-fingered sloths twenty- 

 three or twenty-four, which number is the largest that has been 

 observed in this class. The most common number of the lumbar 

 vertebrge appears to be six or seven; in a few only (the genus 

 Stenops) eight or nine; in many there are five, as in man; only 

 very rarely are there less than four; in the two-toed anteater, the 

 two-toed slotli, and the ornithorhynchus, there are only two. The 

 number of vertebrge of the sacrum is commonly four, but varies 

 from one to nine. In no division of the vertebral column is the 

 number of the component vertebrge subject to greater difference 

 than in the tail, where it varies from four to forty-six. This last 

 number occurs in the long-tailed Mams, in which the tail has 

 nearly three times the length of the rest of the vertebral column. 



The first vertebra has in mammals constantly two articular 

 cavities, for the reception of the two condyles which are found at 

 the sides of the occipital foramen of the skull. This articulation 

 of the skull serves for depressing and raising the head; for rotating 

 it to the right side or the left, there is the articulation of the ring- 

 like first cervical vertebra with the vertical process on the body of 

 the second {ejnstropheus, dens ejjistrojyhei) , which has a smooth 

 articular surface on its anterior side. In this motion the first ver- 

 tebra with the head rolls round the second. In the true whales 

 this tooth-like process is wanting. With them the short neck is 

 immoveable, like the anterior part of the vertebral column in fishes. 

 In many of them also the cervical vertebrge are anchylosed; in the 

 dolphins the first two only have coalesced, and the arches of the 

 five remaining cervical vertebrge are as thin as paper. In the 

 ungulate animals, the carnivora, nay in almost all mammals, with 

 the exception of man and the quadrumana, the first two cervical 

 vertebrge are much larger than the rest; the first has broad, flat 

 transverse processes, which sometimes surpass the breadth of the 

 skull; the second vertebra is long, and its spinous process forms a 

 lengthened ridge-like plate, which extends over the arch of the first 

 cervical vertebra. In many mammals the spinous processes of the 

 cervical vertebrge are conspicuously developed on the second and 

 seventh alone. The spinous processes, on the contrary, of the 

 dorsal vertebrge are commonly long, especially in the ungulate ani- 

 mals. To these processes the cervical ligament [ligamentum nuckce 



