46 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



Including the hypothetical ancestors of the Ichthyosauria the following 

 series of forms may be taken to represent stages in the evolution of the 

 caudal fin : 



1. Unknown littoral or terrestrial Permian ancestor with caudal series much 

 as in Palaeohatteria; tail long, slender, not markedly expanded vertically, 

 upper and lower arches imperfectly developed in posterior part of caudal series. 



2. Unknown semiaqiiatic Permian or early Triassic ancestor. Caudal series 

 long, tail considerably expanded vertically as in Crocodilia, upper and lower 

 arches well developed excepting in terminal third or quarter of tail. 



3. Early Triassic ancestor. Caudal series long, tail expanded vertically 

 somewhat in advance of the posterior end by elongation or erection of the arches. 

 Possibly represented by CymbospondylHs('t') untune or a closely allied form. 



4. Mixosaurus. Middle or Upper Triassic form with distal expansion of 

 caudal region caused in part by elevation of the vertebral arches, and in part 

 by vertical curvature of the vertebral column. 



5. Cymbospondylus. Large Middle Triassic forms possibly represented by 

 preceding stages in the young animal. Distal portion of caudal region retain- 

 ing short Y-shaped chevrons, and short erected neural arches ; vertebral centra 

 increased in height and laterally compressed behind the bend. 



6. Delphinosaurus. Upper Triassic. Expansion of the distal portion of the 

 caudal fin due largely to curvature of the vertebral column. Upper and lower 

 arches considerably reduced in the region of the caudal flexure. Anterior 

 caudal region relatively broad owing to the presence of laterally compressed 

 upper arches of moderate size, and of long-spined chevrons. 



7. Ichthyosaurus. Lower Lias. Much as in Delphinosaurus, but lower 

 arches shortened and no longer united vent rally. 



8. Ichthyosaurus. Upper Lias. As in Lower Lias, but with occasional al- 

 most complete reduction of lower arches. 



9. Ichthyosaurus, Opthalmosaurus, and Baptanodon. Upper Jurassic and 

 Cretaceous. Lower arches much reduced or absent, anterior and posterior 

 margins of vertebrae behind the bend swollen, and with articulation faces 

 swinging outward and somewhat backward on the swollen tracts. Centra be- 

 hind the bend in the tail suddenly reduced in size. (In I. posthumus they be- 

 come relatively small and long.) Posterior portion of caudal series more 

 strongly deflexed than generally in preceding forms. 



As has perhaps been evident in the preceding statement of the evolution 

 of the ichthyosaurian caudal fin, the historical series of forms which is now 

 available suggests that the peculiar upper lobe of the ichthyosaur fin originated 

 at that period in the history of the group when the height of the tail was in- 

 creased by elongation of the neural arches, and that once having taken on an 

 important function it has been retained and enlarged. 



