LIASSIC FORMATIONS. 109 



subservient to the support and movements of the tongue than to the mechanism for 

 defending the larynx and pharynx from the entry of water during the struggles of a 

 submerged prey, when the mouth of the air-breathing destroyer is necessarily exposed to 

 the free ingress of the aquatic medium. The condition of the liyoid apparatus in the 

 Ichthyosaurus indicates that its tongue may have been but little better developed than in 

 the Crocodile, and, since the Ichthyosaur obtained its food under the same circumstances 

 which necessitate the hyoid and lingual modifications in the Crocodile, it may be inferred 

 that the hyoid arch was physiologically related to the working of a similar valvular 

 apparatus for defending the orifice of the air-tube from the water admitted into the 

 mouth during the capture of the fishes, the remains of which have been found in the 

 region of the alimentary canal of the great Sea-lizards. 



The modifications of cranial structure of the known kinds of Ichthyosaurus are 

 chiefly presented by the upper and lower jaws, which become elongated and attenuated 

 in degrees exemplified by the species next to be described. With these modifications are 

 associated increase of number with decrease of size of the teeth, and their total dis- 

 appearance, finally, as in the Ichthyosauroids of the upper Jurassic beds of Wyoming and 

 some other American localities. For these edentulous Ichthysaurs, their discoverer, 

 Prof. Marsh, has proposed the generic name Sauranodon ; ] it is probable that, as in the 

 case of Cetacea, showing minor modifications than do the toothed Ichthyopterygia, other 

 generic terms for some of these species may be proposed. 



C. Species. 



a. Ichthyosaurus breviceps, Ow., Plate XXIX. 



In the skeleton of this species (PI. XXIX, fig. 2) the skull is almost equally divided 

 between the antorbital part and that behind ; it is about one sixth the length of the 

 entire body, as represented by the vertebral column. This includes, in the specimen 

 figured, 125 vertebras, of which 46 lie between the skull and pelvis. The neural spines 

 of such trunk-region are lofty, equalling along its major part the vertical diameter of 

 the rest of the vertebrae taken from the base of the spine. The intervals between the 

 spines are very narrow. The centrums are largest at and near the pelvic region. The 

 fore fin has five normal digital series, with smaller supplementary ossicles along both 

 fore and hind borders ; it is twice as long and as broad as the hind one. 



The specific characters are more fully exemplified in specimens of the skull of larger 

 individuals, which show that the proportions of the rostrum to the rest of the skull in 

 the smaller skeleton may be due to nonage, but the cranial conformation is the same. 



1 ' American Journal oi Science,' vol. xvii, p. 85, January, 1879; ib., vol. xix, p. 1C9, February, 

 1880. 



