MERRIAM: TRIASSIC ICHTHTOSAUEIA. 39 



tween them may disappear or lose its identity. In Ic/iHii/oMuir/ix />fnl//dac- 

 I///IIN as recently described by I>roilf" (fig. 40), not only have the atlas and 

 axis fused, but the third centrum is united with the second. In all of the Ju- 

 rassic ichthyosaurs the centrum of the atlas is nearly or quite as large as the 

 centrum of the axis (fig. 39), while the neuroceutra rest upon it much as the 

 elements of the upper arch of the axis. In this respect the atlas in these forms 

 may be said to be more specialized than in most other reptiles. A possible ex- 

 planation of the peculiar form and large size of the most anterior centrum 

 here is that the anterior cervical centrum of the later ichthyosaurs may cor- 

 respond to the axis of the earlier forms, the atlas having disappeared. 



The known series of forms exhibiting the structure of the anterior portion 

 of the vertebral column of the Ichthyosauria shows pretty clearly that progres- 

 sive modification of the cervical region has taken place along somewhat the 

 same lines that have been followed in the Cetacea. In the whales, fusion of 

 the cervicals has reached an extreme stage, and the neck may be presumed to 

 be much shortened. In view of the considerable difference between the Tri- 

 assic and the Jurassic ichthyosaurs in the form and to some extent in the 

 function of the most anterior centrum, it is possible that in a modification of 

 the cervical region best adapting it to aquatic life, the neck has been short- 

 ened partly by elimination or by extreme reduction of the most anterior 

 elements. 



Fig. 41. Ichthyosaurus acutirostris Owen. Reversed photograph of a specimen in the Carnegie 

 Museum. Reproduced by courtesy of Dr. W. J. Holland. 



Caudal Fin. In no other group of the reptilia have the functions of loco- 

 motion been so largely transferred from the limbs to the tail as we find them 

 to be in the typical ichthyosaur. In consequence of this, the caudal fin of 

 these forms has become the most specialized sculling tail known in the Reptilia. 

 That it has reached the stage of evolution seen in the typical Ichthyosaurus 

 (fig. 41) by passing through grades of development not unlike those of some 



2" Broili, F., Palaeontog., Bd. 54, p. 143. 



