ANATOMY OF VERTEBRATES. 59 



at the fifth. Those of the eighth vertebra first join the sternum, 

 as do those of the ninth and tenth ; the pleurapophyses of the 

 eleventh vertebra suddenly acquire extreme length ; those of the 

 four following vertebras are also long and slender ; they extend 

 outward and backward, and support the parachute formed by the 

 broad lateral fold of the abdominal integuments. The pleurapo- 

 physes of the succeeding vertebras rapidly shorten. The sacrum 

 consists of two vertebras. There are about fifty caudal vertebras. 



The semi-ossified sternum in the Iguana has a median 

 groove and fissure, and readily separates into two lateral 

 moieties. The long stem of the episternum covers the outer 

 part of the groove, where it represents the ' keel ' of the sternum 

 in birds. 



The two sacral vertebras retain, in most Lacertians, the cup- 

 and-ball joints ; and in the Scincks, where they coalesce, the 

 second presents a ball to the first caudal. Hasmapophyses are 

 wanting in the first caudal, commence in the second, but are 

 displaced to the interval between this and the third ; they are 

 confluent at their distal ends, and there produced into a spine : 

 these ' chevron bones ' are continued usually along two-thirds of 

 the tail. In most of the caudal vertebras the anterior third of 

 the centrum is marked off by a line, just anterior to the dia- 

 pophyses, where the tail snaps off, when a lizard escapes, leaving 

 the part that has been seized in the hands of the baffled pursuer. 

 The ossification of the centrum from two points, and their in- 

 complete anchylosis has prospective relation to the liability of 

 lizards to be caught by their long tail, and lends itself to their 

 escape. The epiphysial line does not extend through the thin 

 and brittle neural arch, which readily snaps when the two parts 

 of the centrum to which it is anchylosed are separated. Lizards 

 reproduce the lost tail ; but the vertebral axis is never ossified in 

 the new-formed part. 



In the slow-worm (Anguis) there are 111 vertebras, 6 1 of which, 

 beginning at the fourth, support free ribs. The transverse pro- 

 cesses of the tail are formed by short anchylosed pleurapophyses, 

 which are bifurcate in the second and third caudals. The hypa- 

 pophyses are, also, anchylosed to the centrum ; but, instead of 

 remaining distinct, as in true Ophidia, they unite at their lower 

 ends and complete the haemal arch. The vertebras of the Amphis- 

 bcena have no neural spine. 



The lacertian modifications of the atlas and axis } asree in the 



"&* 



1 xliv. vol. i. pp. 139—149. 



