THE CKOCODILIA. 216 



all crocodiles, and those upon the neck sometimes form dis- 

 tinct " nuchal " and " cervical " groups, distinct from the dor- 

 sal shield. The dorsal scutes do not always overlap, and the 

 ventral scutes are absent, or incompletely ossified, in most ex- 

 isting Grocodilia. 



In these reptiles the vertebral column is always thoroughly 

 ossified, and marked out into distinct cervical, dorsal, lumbar, 

 sacral, and caudal regions. The number of the presacral ver- 

 tebrae is twenty-four; that of the sacral, two, in all the recent 

 forms, and probably in the extinct genera also. The number 

 of the caudal vertebra varies, but is not less than thirty-five. 

 The number of the cervical, dorsal, and lumbar vertebrae varies ; 

 but there are usually nine of the first, eleven or twelve of the 

 second, and four, or three, of the third description. 



lu existing Grocodilia all the vertebra, except the atlas 

 and axis, the two sacrals, and the first caudal, are procoelous. 

 The majority of the pre-cretaceous Grocodilia have the corre- 

 sponding vertebrae amphicoelous, the concavities of the centra 

 being very shallow. One genus, Streptospondylus, which is 

 perhaps Crocodilian, has the anterior vertebrae opisthocoelous. 

 It is characteristic of the Grocodilia, that the centra of the 

 vertebrae are united by fibro-cartilages, and that the neurocen- 

 tral sutures persist for a long time, or throughout life. 



The atlas is composed of four pieces, an upper median 

 piece — which is sometimes divided into two, and is developed 

 in membrane apart from the rest — being added to the three 

 pieces found in Lacertilia and Ghelonia. A large odontoid 

 bone is closely united to, but not anchylosed with, the anterior 

 flat face of the second vertebra. A pair of elongated, single- 

 headed ribs are attached to the inferior piece of the atlas, and 

 another similar pair to the os odontoidum and to the second 

 vertebra, by distinct capitular and tubercular processes. The 

 other cervical vertebra all possess ribs with distinct and long 

 capitula and tubercula — the latter attached above the neuro- 

 central suture to the neural arch, the former to the centrum 

 below the neurocentral suture. The body of each cervical 

 rib, after the second, and as far as the seventh or eighth, is 

 short, and prolonged in front of, as well as behind, the junction 

 of the capitulum with the tuberculum ; and the several ribs 

 lie nearly parallel with the vertebral column, and overlap one 

 another. The ribs of the eighth and ninth cervical vertebrae 

 are longer, and take on more the character of the dorsal ribs, 

 the ninth having a terminal cartilage. 



The points to which the capitula and tubercula of the. ribs 



