436 ON VEKTEBKJE OF POLYPTYCHODON. 



the width of the anterior articular face is 4| inches, its depth is 

 3£ inches. The articular faces become a trifle flatter, and the 

 central elevation rises a trifle higher. The posterior articular face 

 is 7-sided. The articulations for the ribs are now on very slightly 

 elevated pedicles, separated by a narrow margin from the anterior 

 face of the centrum. The facets appear to be flat, nearly circular, 

 and If inch in diameter. The transverse convexity of the base of 

 the centrum increases, while the antero-posterior concavity becomes 

 flatter. 



The antero-posterior measurement of the seventh vertebra is 

 2| inches. The facets for the ribs ascend so high on the sides of the 

 centrum that they must touch the neurapophyses ; hence this would 

 be the last cervical. It is 3j inches deep, and as preserved 

 measures 4-| inches from side to side over the facets for the ribs. 



The remainder of the 16 vertebrae are dorsal ; but they are not well 

 preserved. In these vertebrae the centrum steadily increases in 

 length. Several have an antero-posterior measurement of 2-J- inches ; 

 and one, probably from the middle of the back, is 3 inches long and 

 4 inches deep. The articular faces, though 3f inches wide and 3|- 

 inches deep, look vertically elongated, owing to the centrum being 

 a little compressed from side to side below the neural arch. The 

 margins of the articulations are sharp, and the central boss still 

 remains. The upper part of the centrum leans a little forward as 

 in Pliosaurus. 



I entertain little doubt of the specific distinctness of this fossil ; 

 but since the teeth of animals of this kind rarely furnish specific 

 characters, it will be difficult to prove the separation of the species 

 from Polyptychodon interruptus till better materials of that species 

 are discovered ; and the remains described from the Cambridge 

 Greensand by Professor Owen were not so satisfactory as to yield 

 good characters for comparison. 



The British Museum contains a tooth of Polyptychodon from the 

 Gault. The remains in the British Museum from the Lower Green- 

 sand named Polyptychodon present no characters by which I can 

 recognize their claim to a place in that genus. 



