THE EXXmCT MAMMALIA. 305 



comes rounded ; in the later periods we get them very much 

 rounded ; and, finally, in the latest forms, we get the double 

 curve and the locking process in the vertebral column, which, 

 as in the limb, secures the greatest strength with the greatest 

 mobility. 



In the first stages of the growth of the spinal column, it is a 

 notochord, or a cylinder of cartilage or softer material. In later 

 stages the bony deposit is made in its sheath until it is perfectly 

 segmented. Now, all the Permian laud-animals, reptiles and 

 batrachians, retain this notochord with the elements of osseous 

 vertebrae, in a greater or less degree of completeness. Tliere are 

 some in South Africa, I believe, in which the ossification has come 

 clear through the notochord ; but they are few. In this charac- 

 teristic the Permian appears almost, perhaps absolutely, peculiar as 



Fig. 61. — Sleeve of a coat showing folds produced by lateral flexure whicli leaves 

 interspaces similar to the segments of a rhachitomous vertebra. Thus, i represents in- 

 tercentrum ; p, pleuro-centrum ; and n, neurapophysis. 



regards land-aniraals. There is something to be said as to the con- 

 dition of the column from a mechanical standpoint, and it is this : 

 that the chorda exists, with its osseous elements disposed about it ; 

 and in the Permian batrachians, equally related to salamanders 

 and frogs, these osseous elements are arranged in the sheath or 

 skin of the chorda ; and they are in the form of regular concave 

 segments, very much like such segments as you can take from the 

 skin of an orange — but parts of a cylinder, and having greater or 

 less dimensions according to the group or species. Now, the point 

 of divergence of these segments is on the side of the column. The 

 contacts are placed on the side of the column where the segments 

 separate — the upper segments rising and the lower segments com- 

 ing downward. To the upper segments are attached the arches 

 and tlieir articulations, and the lower segments are like thd seg- 

 ments of a cylinder. If you take a flexible cylinder, and cover it 

 with a more or less inflexible skin or sheath, and bend that cylinder 



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