1887.] On Parieasaurus bombidens {Owen). 341 



in the two groups are identical. Similarly from the structure of 

 Parieasaurus and its affinities to Mesosaurus, it is argued that the 

 Nothosauria and Plesiosauria are closely related ; and since they have 

 the long bones ossified as in existing Amphibians, with the biconcave 

 vertebra?, double-headed cervical ribs on the centrum (in many genera), 

 with dorsal ribs rising to the neural arch, and much in common in 

 the clavicular and pelvic arches, it is held to be a legitimate inference 

 that both the JSTothosauria and Plesiosauria have undergone, with the 

 acquisition of the basi-occipital bone, a loss of Labyrinthodont charac- 

 ters, which has removed from the skull every technical trace of the 

 group from which both of those orders are shown to have been derived,, 

 by the Amphibian and Labyrinthodont characters which they have pre- 

 served in other parts of the skeleton. Similarly it is concluded that the 

 Orocodilia derive their typical characters from the Labyrinthodontia. 

 The crocodilian skull preserves the anterior position of the nares and 

 small size of the pre- maxillary bones. The temporal fossa? are sub- 

 stantially similar, and the unossified condition of the post-orbital 

 membrane in crocodiles accounts for the absence of the post- 

 orbital and supra-temporal bones between the post-frontal and squa- 

 mosal bones above, and the jugal and quadrato-jugal bones below. 

 While if those ossifications were developed in the vacuity behind the 

 eye the quadrate would be completely hidden, and the skull would be 

 externally as Labyrinthodont as that of Ichthyosaurus. The number 

 of pre-sacral vertebras in crocodiles shows no great divergence from 

 Parieasaurus ; while the mode of articulation of the dorsal ribs in 

 Parieasaurus, although not quite crocodilian, make a nearer approach 

 to the crocodilian type than is seen among other Reptilia, and is so 

 similar as to justify the conclusion which the skull suggests — that 

 Crocodilia have been modified from a Labyrinthodont ancestry. If 

 this conclusion is admitted for the Crocodilia, it follows for the 

 Dinosauria also, since that group is essentially a parallel variation 

 from the crocodilian type. The Profcerosauria are already known to 

 show many Labyrinthodont characters, combined with those of reptiles 

 and mammals ; and the similar Ornithosaurian pelvis is essentially a 

 variation from that of the Anomodont, and would thus be an inherit- 

 ance of structures which were of Labyrinthodont origin. Hence the^ 

 Reptilia are to be regarded as related to each other as descendants 

 from a common and varied ancestral type ; and, therefore, the orders 

 should be grouped in parallel rather than vertical or successive rela- 

 tion to each other. 



The mammalian characters of the pelvis and sacrum of Pariea- 

 saurus and the other Anomodonts are quite as striking as the Avian 

 characters of certain Dinosaurs, and of the same kind of importance 

 as evidence of affinity. If the community of structure of Iguano- 

 donts and Birds is held to establish a common origin for both groups, 



