1898] STRUCTURE AND HABITS OF PLESIOSAURIA 51 



head iu a long porpoise-like snout. The principal power of locomo- 

 tion in the Plesiosauria resided in the limbs, as is indeed evident 

 from their remarkable length in proportion to the trunk. The fore 

 and hind extremities were also nearly of equal size. That the latter 

 were as powerful for locomotion as the former is shown by the 

 structure of the pelvis, which, with its great spreading pubis and 

 ischia, afforded space for the attachment of large muscles, just as 

 the coracoids in front provided for the fore limbs. Here also there 

 is an important difference from the Ichthyosauria, whose pelvic 

 elements are reduced to delicate rod-shaped bones, corresponding 

 with the small hind limbs. Further, while the Ichthyosauria relied 

 on the large downwardly heterocercal tail fin for a good part of their 

 locomotion, as recent discoveries in Swabia have shown, the tail fin 

 of the Plesiosauria was less suited for that purpose on account of 

 its smaller expansion. The frequently enlarged neural arches of the 

 last caudal vertebrae indicate that bands of muscle extended into the 

 fin, so that it would admirably serve as a balancing and steering 

 apparatus. 



The accompanying drawing (Plate III.) represents two Plesiosauri 

 as they may have appeared according to the conclusions here 

 advanced in reference to their shape and habits. The one individual 

 is swimming below the surface of the sea in search of prey, the other 

 is rising to the surface to breathe. They are represented of about 

 one-twentieth the natural size. W. Dames. 



University of Berlin. 



