480 ICHTHYOSAURIA CHAP. 



the folds of the Labyrinthodonts. The teeth have open roots, 

 and are not implanted in separate alveoli, but lie in long 

 grooves of the premaxillaries, maxillaries, and dentals. 



The vertebrae are numerous, up to 150, two-thirds of which 

 belong to the tail. The centra are deeply biconcave and short, 

 not co-ossified with the neural arches, which have therefore often 

 broken loose. The atlas much resembles the other cervical verte- 

 brae in so far as its centrum is concave in front and scarcely 

 ankylosed with that of the second. Its basiventrals, equivalent 

 to the ventral half of the atlas-ring of other reptiles, thus become 

 an unpaired intercentral wedge, between the first centrum and the 

 basis of the cranium ; the neural arches rest upon the centrum, 

 but remain separate from each other, or at least diverge dorsally. 

 The atlas carries no ribs. Intercentra occur also between the 

 second and third vertebrae ; they reappear in the tail as chevron- 

 bones. All the other vertebrae carry ribs, which gradually 

 increase in length towards the trunk and decrease again equally 

 gradually on the tail. In the neck and trunk they have separate 

 capitula and tubercula, which articulate upon short knobs of the 

 centra ; towards the tail these shift farther and farther towards 

 the ventral side, and ultimately unite. Although the ribs of the 

 trunk are so long, there is no trace of a sternum, but there are 

 many " abdominal ribs " crowded together, each consisting of a 

 middle and a pair of lateral pieces. 



The shoulder-girdle is very complete, but the pieces remain 

 separate, or at least do not co-ossify; it consists of a T-shaped inter- 

 clavicle, clavicles, broad coracoids touching each other in the 

 middle line, and short scapulae. The existence of small separate 

 precoracoids is doubtful. The pelvis is much reduced ; the small 

 ilium is quite unconnected with any vertebrae ; the small pubes 

 and ischia form no symphyses. The fore- and hind-limbs are 

 very similar to each other ; the posterior are, however, much 

 smaller. Both are transformed into highly specialised paddles. 

 It is of the greatest importance, as an indication that the 

 Ichthyosauri are descendants of a terrestrial stock, and have been 

 modified into what they are owing to having taken to marine 

 life, that in the oldest members known, the paddle-like structure 

 of the limbs was less advanced than in the later species. In 

 Mixosaurus of the Muschelkalk of Europe the ulna and radius 

 are still distinctly longer than broad, and they enclose a space 



