434 H. GOVJER SEELEY OX AN ASSOCIATED SERIES OP 



The superior lateral angles of the atlantal cup show some indica- 

 tions of large flattened surfaces, which look obliquely forward and 

 upward and somewhat outward, and encroach on the cup ; from 

 these the neural arch has come away. The inferior and outer 

 corners of the cup are greatly abraded ; but they show no indications 

 of having been formed by distinct subvertebral ossifications. 



The posterior articular face appears to be smaller. It is flattened, 

 slightly concave from above downward, and more concave from side 

 to side. As preserved, the axis-face measures 3J inches in depth 

 and 4 inches in transverse diameter. 



The base of the specimen is slightly convex in vertical outline, 

 and less than 4 inches in length between the facets on the axis for 

 the cervical ribs. 



The side of the fossil is formed of two areas : — first, a superior 

 surface 2\ inches deep, on each side of the vertebrae ; these con- 

 verge somewhat upward towards the neural arch ; and secondly, 

 a smaller inferior impressed surface on each side, 1^ inch in 

 diameter, which looks backward and downward, and gave attach- 

 ment to the first cervical rib. This area was defined and separated 

 by ridges from the unarticular side of the centrum above, and from 

 its base beneath. 



The length of the specimen from front to back was greater at tho 

 base of the centrum than at the neural arch. Its greatest antero- 

 posterior measurement is nearly 2j inches, which is more than in 

 the species called by Professor Owen Polyptychodon interruptus. 



The least antero-posterior measurement through the centres of 

 the articular cups is 1 j inch ; the antero-posterior measurement 

 below the neural canal as preserved is rather over 2 inches. 



The basal or visceral surface of the atlas and axis is slightly con- 

 vex in length, slightly concave from front to back, and shows 

 rounded nutritive foramina at a distance of 2 inches from each 

 other, and 1 inch from the postarticular border. 



Other Cervical Vertebrce. 



I have arranged the five succeeding vertebras in natural sequence. 

 They are remarkable for a Pliosauroid antero-posterior shortness, 

 and a Pliosauroid flatness of the articular surfaces ; but they differ 

 from the cervical vertebras of Pliosaurs hitherto described in having 

 a single articular facet on the centrum for the cervical rib, instead 

 of two facets ; and this character is the only one so far as I am 

 aware by which a short-necked member of the Plesiosaur family 

 can be distinguished from a short-necked Pliosaur when only 

 centrums of vertebras are available for comparison. The facets for 

 the ribs on the side of the centrum gradually ascend the side of the 

 centrum from the neck backwards, after the manner of Plesiosaurs, 

 the ribs evidently becoming supported on the transverse processes 

 of the neural arch in the dorsal region ; so that Polyptychodon is 

 essentially a big-headed Plesiosaur with no characters at present 

 discovered to separate it from other Plesiosaurs, except the not very 



