G6 BRITISH FOSSIL REPTILES. 



extremity of the mandible tlie jaw of IclilliyomurHS more nearly resembles that of the 

 Crocodiles than of the feebler Lizards, bnt in the mandibular structures indicative of 

 affinity these latter existing Reptiles manifest their closer connection with the Ichthyo- 

 saurus. This is conspicuously seen in the absence of distinct alveoli and the lodgment 

 of the teeth of Ijoth upper and lower jaws in a continuous open channel, the inner wall 

 of which, in the mandible, is in a large proportion contributed by the splenial element. 

 But the cement-clad base or root of the tooth seems not to become anchylosed to the 

 alveolar tract or groove in Ichthyosaurus, but to remain free, till shed, as in Crocodiles. 

 Although a portion of the pul[)-cavity may persist in the fully developed tooth after the 

 base or root becomes consolidated by a mass of interblended osteodentine and cement, into 

 this mass the crown of the successional tooth presses, and occasions a cavity by absorption.' 

 In no case have I found evidence of this successive supply of new teeth in the Triassic 

 or Permian Theriodonts : herein differs their dentition from both Crocodilian, Dinosaurian, 

 Lacertian, and Enaliosaurian Reptilia. 



In most Lizards the hyoid bones present modifications which relate to the size and 

 uses of the thick, or long, and commonly bifurcate, tongue. In Ichthyosaurus the appa- 

 ratus is reduced to the same number of pieces as in the Crocodile, in which it is less 

 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 hyoid 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 Ichthysam's, their discoverer. 

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



1 ' Catalogue of the Fossil Reptilia and Pisces iu the Museum of the College of Surgeons,' -Ito, 1854, 

 p. 40, Nos. 139, 140. In No. 141 I point out that in some of the teeth " the pulp-cavity has been 

 obliterated in the crown as well as in the base of the tooth." See ' Quarterly Journal of the Geological 

 Society' for May, 18/9, pp. 189 and 199. 



2 ' American Journal of Science,' vol. xvii, p. 85, January, 1879 ; ib., vol. xi.\, p. 169, February, 1880. 



