VEETEBE.E OF VEETEBEATA. 437 



dorso-lumbar vertebras. Iu the Marsupialia and Artiodactyla this is 

 always 19 ; the same number, or 20, predominates in most Rodents, 

 Carnivora (21 iu Paradoxurus and Procyon), and the majority of 

 Primates. In some of the latter, however, it falls to 17 or 18, iu 

 which number they agree with most of the Chiroptera. 



The variations within the dorso-lumbar region depend on the 

 ribs, for when they undergo degeneration the lumbar vertebras are 

 increased in number. 



§334. 



Varied as are the differentiations undergone by the vertebras, the 

 extreme conditions are usually connected by intermediate forms. It 

 is in the first two vertebras only that an arrangement obtains 

 which is limited to them, and them only; and this arrangement is 

 due to the way in which the skull is connected with, and moves on, 

 the spinal column. 



In Fishes we sometimes meet with peculiar arrangements in 

 the way in which the skull and first vertebra are connected 

 together ; in the Rays there are articular facets, as there are also in 

 the Teleostei, where they are placed on lateral processes. Another 

 set of modifications commences in the Amphibia. The first cervical 

 vertebra is circular, as it generally has no transverse processes, 

 which are only present when it is fused with the succeeding vertebra 

 (Pipa). This first vertebra is known as the atlas. In the Reptilia 

 the centrum of the atlas is separated from its arch, and is placed in 

 front of the centrum of the second, which is distinguished as the 

 axis, and it is more closely connected with the centrum of 

 the axis than with its own arch. Owing to this arrangement 

 a special piece is developed, which connects the two sides of the arch 

 on the ventral surface. In the Crocodilini the arch is also closed by 

 bone on its dorsal surface. In the Ophidii, the part which cor- 

 responds to the centrum of the atlas fuses with the second cervical 

 vertebra, and forms its odontoid process. The same arrangement 

 obtains in Birds, w T here the ventral connection of the arch becomes 

 very large indeed as compared to this " odontoid process." 



The arrangement seen in the Reptilia is represented in an em- 

 bryonic stage in Mammalia. In the Monotremata it lasts some time 

 longer than in the rest. In the Marsupialia, however, it is often a 

 permanent condition, owing to the separation of the centrum of the 

 atlas from the axis. In other cases the centrum of the, atlas passes 

 completely into the odontoid process of the axis. The inferior union 

 of the arches is, in the Marsupialia, merely represented by a liga- 

 ment, or there may be a distinct bone in its place. In the Mono- 

 delphia there is a bony ventral clasp between the two halves of the 

 arch. 



