64 BRITISH FOSSIL REPTILES. 



to join the ischium at its lower end ; the opposed borders of the two Ijones are concave, 

 and inclose a long and large ' obturator ' vacuity. 



Modifications of these pelvic elements are shown in Ichthvosaurus communis and 

 Ich. latifrons, and will be noticed in some other species. 



The femur (Tab. XXVII, fig. 1, 65) resembles the humerus, but is smaller and usually 

 rather less broad. Its distal end supports two cnemial bones, repeating the shape and 

 relative proportions of the antibracliial ones. To these succeed three tarsal ossicles, which 

 support four series of digital ossicles. But in Ich. communis the anterior or tibial series 

 soon bifurcates, and supplementary ossicles are applied to those of the fibular or hinder 

 digit. The shape of the ventral or pelvic fin thus supported is similar to, but relatively 

 less broad (in Ich. communis) than, the pectoral one. 



The rare illustration given in Tab. XXIV, fig. 5, is of a pelvic fin. 



d. Osteohgical Summary. — At the risk of some repetition the following remarks, 

 the result of such comparisons as I have had time or means of extending over the cold- 

 blooded air-breathers, may not be unacceptable. The comparison with existing Reptilia 

 is almost restricted to the Lacertian and Crocodilian modifications. 



In Crocodiles the exoccipitals leave an intervening tract of the basioccipital, which 

 thus takes its share in support of the ' medulla oblongata'; in Lizards the exoccipitals meet 

 upon the basilar element, as in Ichthyosaurus, but the extinct Reptile differs from both 

 in the exclusive formation, by the basioccipital, of the condyle articulating with the 

 atlas. Such joint in Eishes is afforded by the basioccipital only, but the articular surface, 

 as a rule, is concave instead of convex. The superoccipital is excluded from the foramen 

 magnum in Crocodiles, but contributes, in Lizards, a share thereto, as in Ichthyosaurus. 

 The paroccipital is confluent with the exoccipital in both Crocodiles and Lizards, as it is 

 in Plesiosaurs.^ It remains distinct in Chelonians as in Ichthyosaurs. 



The extension of the mastoid upon the occipital region of the skull gives it an aspect 

 of solidity more like that in Crocodiles than in Lizards ; but this is an adaptive con- 

 formation, and depends on the need of an extent of bony surface for the implantation of 

 the powerful nuchal muscles mainly concerned in wielding a head produced into long 

 and heavy jaws, beset, as a rule, with formidable teeth ; it also relates to the stabihty of 

 the prow of the Fish-lizard in cleaving the watery element. The occipital aspect of the 

 diverging extensions of the parietals, and the presence of the ' fontanelle,' called 

 ' foramen parietale ' in Plesiosaurs" as well as Ichthyosaurs, are more decisive instances 

 of the closer affinity of Lizards, than of Crocodiles, to the antecedent marine types of 

 Beptilia. No part of the parietal extends upon the occiput in Crocodiles, but this 

 extension holds in Plesiosaurs as in Ichthyosaurs. 



In adaptive relation to the mandible and its armature I regard the relative size and 



1 Ante, p. 8. 



2 lb.. Tab. IV, fig. 1,7. 



