514 PLESIOSAURIA. 



tebrae and the proportions of the limbs, which the palaeontologist does 

 not regard as generic characters. Pelagosaurus is distinguished by the 

 interspace between the eyes being greater than the diameter of the eye ; 

 it is characteristic of the Lias. The name Teleosaurtis has been 

 reserved for the crocodiles of the Inferior and Bath Oolite, which 

 have large and approximating orbits. The Teleosaurs of the Kimme- 

 ridge Clay have the eyes lateral, and have been referred to the genus 

 Steneosaurus. Gnathosaurus of the lithographic slate is a crocodile 

 with lateral eyes, of doubtful affinity. In the Purbeck and Wealdon 

 beds some crocodiles occur which in cranial characters make a transi- 

 tion towards the modern type. They comprise Goniopholu, Petro- 

 suchus, and Theriosuchus. 



The Eucrocodilia or Proccelia appear for the first time in the Cam- 

 bridge Greensand, Gosau beds, and Greensand of New Jersey. They 

 are met with in the London Clay and Lower and Middle Tertiaries 

 of Europe. The gavial is found in the Bracklesham beds, and the alli- 

 gator in the Headon beds. 



Plesiosauria. The Plesiosauria comprise a variety of saurians. 

 They include two principal groups the Nothosauria of the Trias and 

 the Plesiosauria which range from the Rhsetic beds to the Chalk. 

 These groups are linked together by the triassic genus Neusticosaurus, 

 which exhibits a land animal in process of acquiring the aquatic limbs 

 of the plesiosaui. 1 At first sight, plesiosaurs offer many characters in 

 common with chelonians, such as the forms of the pelvic bones and 

 pectoral bones, and even in the shape of the larger-limb bones, espe- 

 cially as seen in the Nothosauria. But as the anatomy is examined 

 in detail, there are many suggestions of affinity to crocodiles. All 

 plesiosaurs have the palate closed, and the teeth in distinct sockets, 

 the eyes midway in the side of the head, usually a long neck, the cer- 

 vical ribs occasionally have the articulation double, frequently single. 

 Elasmosaurus has no interclavicle. The terrestrial plesiosaurs become 

 greatly modified, and apparently develop into the Anomodontia. The 

 British fossil genera are at present only defined in a few instances ; 

 they include the large-headed form with triangular teeth and short 

 cervical vertebrae named Pliosaurus, ranging from the Kimmeridge 

 Clay to the Neocomian beds. In the Oxford Clay there is a long- 

 necked type called Murcenosaurus, with an imperfectly developed pec- 

 toral arch, and in the Kimmeridge Clay another Elasmosaurian type 

 named Colymbosaurus. There is a stiff-backed plesiosaur called Stcreo- 

 saurus in the Cambridge Greensand, and Polyptychodon is a genus of 

 the Greensand and Chalk. All these genera are remarkable for the 

 uniform character of the vertebrae, which are moderately cupped (ex- 

 cept in Stereosaurus, where they are flat), and in the dorsal region have 

 the ribs supported upon the neural arch. 2 



Anomodontia. The Anomodontia have the palate more open 



1 See Monographs on Sauropterygia and Ichthyopterygia by R. Owen, F.R.S., 

 published by Palseontographical Society. 



2 Dr. Liitken regards Lariosaurus as a Plesiosaur. See his valuable " Skild- 

 ringer af Dyrelivet i Fortid og Nutid." Kjobenhavn. 1880., 



