108 FOSSIL REPTILIA OF THE 



Both Lacertians and Crocodilian s differ from Ichthyosaurs in the connections of 

 the nasal with the maxillary. The Crocodiles resemble them in the inter-pre- 

 maxillary suture; its presence is an exception in Lacertians, the Rhynchosaurians and 

 Rhynchocephalians 1 again affording such examples; I have found it obliterated in a 

 Plesiosaurus dolichodeirus* In the position and formation of the palatal nostrils the 

 Lacertians agree with, whilst the Crocodiles widely depart from, the Ichthyosaurian 

 type. The apertures are distinct or parial in the Plesiosaurs, but are placed far back. 3 



In the structure of the mandible the dentary resembles that element in Lizards, and 

 differs from that in Crocodiles, in being pierced externally by a longitudinal series of 

 nervovascular foramina ; it differs, also, from the dentary in Crocodiles in its posterior 

 termination being above instead of beneath the fore end of the surangular. In the 

 amphiccelian Crocodiles the vacuity between the angular and surangular is much 

 reduced in size; it is still smaller in Rhynchocephalians ; it is absent in Ichthyosaurs, as 

 in Plesiosaurs 4 and most Lizards. 



In the conformation of the posterior angle and the robustness of the articular 

 extremity of the mandible the jaw of Ichthyosaurus more nearly resembles that of the 

 Crocodiles than of the feebler Lizards, but 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 both 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 pulp-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. 5 

 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 



1 Monogr. cit., pi. xvi, fig. 1, 22. "On the Affinities of Rhynchosaurus," 'Annals and Mag. of Nat. 

 History,' iv, 1859, p. 237. 



2 Monogr. cit., pi. iii, fig. 1. 



3 lb., pi. xvi, fig. 2, r, r. 



4 Monogr. cit., pi. ii. 



5 ' Catalogue of the Fossil Reptilia and Pisces in the Museum of the College of Surgeons,' 4to, 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, 1879, pp. 189 and 199. 



