VERTEBRA OF CONSTRICTING SERPENT. 73 



serpents. The neural canal is surrounded by a neural arch which is anchy- 

 losed to the centrum as it is in L acertilia and in Chelonia with few exceptions. 

 The neural spine is low. At its base anteriorly, there stands above the 

 neural canal a bony wedge, the zygosphene, with articular surfaces looking 

 obliquely outwards and downwards. In a similar position but posterior to 

 the spine is a deep cavity, the zygantrum, with articular surfaces looking 

 obliquely inwards and upwards. The wedge and cavity fit the one into the 

 other in contiguous vertebrae. Similar structures are found in the Iguana 

 among Lizards, and the Edentata with the exception of the Sloths among 

 Mammalia. The articulating processes are remarkably large and flat The 

 anterior pair (prae-zygapophyses), placed externally to the zygosphene, 

 have their surfaces disposed typically, i. e. looking obliquely inwards and 

 upwards, relations reversed in the posterior pair (post-zygapophyses). A 

 low ridge connects the prae- to the post-zygapophysis of the same side. 



The size of the articular surfaces, their disposition, the depth of the 

 cup and of the zygantrum, and prominence of the ball and zygosphene 

 permit great freedom of motion and at the same time prevent dislocation. 

 The fact that the cup and zygosphene are both anterior, and the ball and 

 zygantrum posterior, constitutes a further safeguard in the same direction. 



Below the prae-zygapophysis lies the diapophysis or articulating surface 

 for the rib. Its upper portion is convex in every direction, while the lower 

 portion is concave from above downwards but convex from before back- 

 wards. This lower portion in some Snakes, e.g. Rattlesnake, is much pro- 

 longed ventrally. 



The atlas resembles the corresponding vertebra in Lacertilta and Chelonia. 

 It consists of three pieces : one inferior, prolonged ventrally into a spine ; and two, 

 one on either side, forming the neural arches. There is no neural spine. The 

 odontoid process is united by anchylosis to the centrum of the axis. It carries an 

 inferior spine united to it, at least in young specimens, by a suture and representing 

 perhaps a sub-vertebral wedge-bone such as exists in many Lizards between the 

 centra of two adjoining vertebrae. There are no ribs to the atlas and axis, but in 

 a specimen of Python (sp. ?) in the Oxford Museum cartilaginous representatives of 

 these structures exist The anterior caudal vertebrae, to the number sometimes of 

 ten but never more, have ribs apparently forked at their vertebral end. The ventral 

 division of the fork is perhaps an outgrowth from the centrum ; i. e. represents the 

 lower transverse process (= parapophysis). The lymphatic heart is lodged in the 

 space enclosed by the fork. 



Vertebral column. Owen and Bell, Reptilia of the London Clay, Part iii. 

 Palaeontographical Society, 1850; De Rochebrune, Journal de 1' Anatomic, &c. 

 (Robin), 17, 1 88 1. 



Skull of Common Snake. W. K. Parker, Ph. Tr. 169, 1878. 



