74 



MEMOIRS OF THE UNIVERSITY OP CALIFORNIA. 



comparing the oldest forms with the latest there is, however, a strong contrast 

 in the structure of man} 7 of the important parts of the skeleton, and the dif- 

 ferences are of such a nature and so graded in time as to suggest progressive 

 specialization of a very definite type. 



A list of characteristics in which the Triassic and the later Jurassic ichthy- 

 osaurs are found to differ would include the following: 



LATER JURASSIC FORMS. 



1. Locomotion almost solely by means of 



powerful vertical caudal fin. 



2. Limbs much abbreviated, and functioning 



largely as fins rather than as paddles. 

 Epipodials reduced to the form of mes- 

 opodials. 



3. Hind limbs relatively small. 



4. Epipodial, metapodial, and phalangeal ele- 



ments reduced to polygonal plates, or 

 to approximately circular discs with 

 concave margins. 



5. Number of phalanges in each digit greatly 



increased. 



6. Number of digits often greater than five. 



7. Pelvis much reduced : in later forms with 



ischium and pubis fused. 



8. Pelvis loosely connected with body. 



9. Skull much elongated, and relatively large 

 compared with the trunk. 



10. Jaws slender and much elongated. 



11. Maxillaries relatively small ; premaxillaries 



relatively large. 



12. Orbits greatly enlarged. Superior tem- 



poral opening small, bar behind orbit 

 very narrow. 



13. Auditory apparatus consisting of massive 



elements adapted to use in deep diving. 



14. Teeth numerous, simple-conical, loosely at- 



tached. 



15. Vertebral centra high, relatively short an- 



teroposteriorly. 



TRIASSIC FORMS. 



1. Locomotion assisted to a greater extent by 



paddles than in the Jurassic forms. 



2. Limbs relatively large and with important 



function as paddles. First two seg- 

 ments relatively long. 



3. Hind limbs relatively large. 



4. Epipodial, metapodial, and phalangeal 



elements often elongated, and with 

 shafted median region. 



5. Number of phalanges in each digit rela- 



tively small. 



6. Number of digits not increased beyond 



five. 



7. Pelvic elements heavy: inferior elements 



much expanded. Ischium and pubis 

 never fused. 



8. Upper end of ilium broad, indicating some- 



what firmer connection with vertebral 

 column than in Ichthyosaurus. 



9. Skull shorter compared with length of the 



trunk. 



10. Jaws generally relatively short. 



11. Maxillaries relatively long, and premaxil- 



laries relatively short. 



12. Orbits relatively small, temporal bar be- 



hind orbit broad. 



13. Element supposed to represent stapes rela- 



tively small. 



14. Teeth numerous in Cymborpondylus, 



relatively few in Mlxosanrus. Teeth 

 set in pits in older forms. 



15. Vertebral centra averaging relatively long 



and low. 



