SKELETON. 



373 



lumbar, sacral, and caudal regions (Fig. 

 202). There are 24 or 25 presacral 

 vertebrae, 35 or more caudal and two 

 sacral. The presacral vertebrae are 

 usually distributed as follows : nine 

 cervical, eleven or twelve dorsal, and 

 three or four lumbar. In living croco- 

 diles and in extinct species from the 

 Cretaceous onwards, all the vertebrae 

 except the atlas and axis (epistropheus), 

 the second sacral, and the first cau- 

 dal, are procoelous. The second sacral 

 is flat in front and concave behind, 

 and the first caudal is biconvex. In 

 precretaceous forms the centra are 

 feebly amphicoelous. The centra are 

 united by discs of fibro-cartilage and 

 the neurocentral suture persists. 



All the cervical vertebrae have ribs. The 

 atlas consists of four pieces, an unpaired ven- 

 tral, two lateral, and a dorsal. The ventral 

 piece carries a pair of backwardly project- 

 ing single-headed ribs ; the dorsa^ piece is 

 developed in membrane and has ueen inter- 

 preted as the remnant of the neural arch of 

 a vanished proatlas vertebra. The axis has C/; 

 an odontoid process, which is suturally joined 

 to it. A two-headed rib is attached to the 

 odontoid process. The other cervical ver- 

 tebrae all possess two-headed overlapping 

 ribs, of which the capituliun articulates with 

 a facet or small tubercle on the centrum, 

 and the tuberculum with a short transverse 

 process projecting from the neural arch 

 above the neiu'o-central sutvire. In the 

 first and second dorsal vertebrae the capi- 

 tvilar tubercle is on the neurocentral suture, 

 and in the third dorsal it has risen above 

 this and is on the transverse process. In 

 the succeeding vertebrae the point of at- 

 tachinent of the capitulum gradually moves 

 outwards until it becomes joined to the tuber- 

 cular attachment, and there is only one at- v<,„ .:>n> ^.i i ^ 

 + , , r ^, ., ^ ^, , ^ , ■* 10. 202. ^Skeleton of Crocodile. C 



tachment for tlxe rib, at the end of the first caudal vertebra ; D dorsal 

 transverse process. The lumbar vertebrae 't'^^racic) region ; F fibula ; Fe 

 „,.„.,, 5 •. J , , ^ femur ; H humerus ; J ischium ; 



■aie without ribs and have long transverse i lumbar region ; R radius; m 



ribs ; Sa sacral region ; Sc scapula; 

 Sta abdominal ribs ; T tibia ; U 

 ulna. 



