88 WATER REPTILES OF THE PAST AND PRESENT 



immovable. The lower jaws, always rather slender, are firmly 

 united in front, sometimes for a long distance, as in the modern 

 gavials. The teeth of the broad-headed plesiosaurs are long, 

 slender, pointed, and recurved, of a murderously cruel shape; they 

 are deeply implanted in sockets, and number from twenty to 

 thirty on each jaw above and below. There are no teeth on the 

 bones of the palate, such as the mosasaurs possessed. The slender- 

 jawed, gavial-like plesiosaurs have more numerous, but smaller 

 teeth. The surface of the skull on each side behind, for the attach- 

 ment of the muscles closing the mandibles, is of great extent; in 

 some this surface is increased by a high, thin crest in the middle, 

 as in strongly carnivorous animals, all of which give conclusive 

 evidence of the powerful muscles used in biting and seizing. There 

 is but one temporal opening on each side, as in the ichthyosaurs 



Fig. 41. — Skull of Trinacromerum from the side: ang, angular; d, dentary; pm, 

 premaxilla; po, postorbital; /, jugal; sur, surangular. 



and the mosasaurs, whereas the crocodiles, thalattosaurs, phyto- 

 saurs, etc., have two. The brain cavity of all plesiosaurs is small, 

 though the cavities of the internal ears, the semicircular canals at 

 least, are large. The semicircular canals in vertebrates have little 

 or nothing to do with the function of hearing; they serve rather for 

 equilibration, for the co-ordination of muscular movement; possi- 

 bly we may infer from their large size in the plesiosaurs that they 

 were not at all clumsy in their movements. There is a large open- 

 ing for the pineal body, the so-called eye in the roof of the brain 

 cavity, though its possession does not necessarily imply the pos- 

 session of a functional organ. 



The Plesiosauria included some of the largest aquatic reptiles 

 that have ever existed, equaled, perhaps, though not exceeded, 

 by some of the extinct crocodiles. The largest known are probably 



