ON A NEW SPECIES OF PLESIOSAURUS S3 1 



Tlie Structure of the Atlas and Axis. 



Thanks to the investigations of Sir Philip Egerton, the structure 

 ■of the axis and atlas of Iclithyosaiirus is placed beyond doubt. But 

 our knowledge of the corresponding parts of P lesiosaurus cannot be 

 said to be by any means so well based, since it rests, so far as I am 

 aware, upon the examination of a single and imperfect specimen, 

 which has been described in the following terms by Professor 

 Owen : — 



" A recent opportunity of examining the atlas and axis of the 

 P lesiosaurus, kindly afforded me by my friend Professor Sedgwick, 

 has not only strengthened this view of the general nature of the 

 " subvertebral wedge-bones,' but has made me incline to the second 

 hypothesis of the special homology of the first or anterior of the wedge- 

 bones which is proposed in my ' Report on British Fossil Reptiles,' 

 viz. : — That it answered to the part described as the body of the atlas, 

 in the existing Saurians and Chelonians ; which therefore may be 

 regarded, like the first subvertebral wedge-bone, as the cortical part 

 only of such vertebral body, like the plate of bone beneath the bicon- 

 cave central part of the body of the atlas in the Siluroid fish. 



" The atlas and axis in the Plesiosaiirus (fig. 3) preserve the general 

 proportions of the other cervical vertebrae, and are consequently longer 

 than their homologues in the Ichthyosajirus ; but they are similarly 

 ■anchylosed together, and measure \\ centimetres (nearly 2 inches) in 

 length, 3 centimetres across the anterior concave surface of the atlas, 

 and '})\ centimetres across the less concave posterior surface : the 

 neural arch of each vertebra has coalesced with its centrum, and a 

 long obtuse process is formed below by a similar coalescence of the 

 first and second wedge-bones with each other and their respective 

 centrums. The limits of the anterior wedge-bone, ca, ex, are trace- 

 able : it is proportionally larger thaij in the Ichthyosaurus (fig. 2), in 

 which it is likewise larger than the succeeding wedge-bones. It forms 

 in the Plesiosaurus the lower third part of the atlantal cup for the 

 ■occipital condyle B ca, ex: the anchylosed bases of the neurapophyses 

 •(« a) form the upper border of the cup, and the intermediate part or 

 bottom of the cavity is formed by the centrum of the atlas (ca), or 

 rather by that part which, like the biconcave centrum in the Siluroid 

 fish, is developed from the central portion of the notochord. 



"The smaller or second wedge-bone {ex, ex^ is lodged in the infe- 

 rior interspace between the atlas and axis, but has coalesced with both 

 ,bones, as well as with the large anterior wedge-bone or cortical part 



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