ANATOMY OF VERTEBRATES. 51 



wedged between the atlas and axis, a third between this and the 

 third vertebra ; all tending to strengthen and stiffen the part of 

 the vertebral column sustaining the skull, and adding to its power 

 of displacing the water in the agile movements of this ancient 

 predatory aquatic animal. 1 As in Fishes, also, the continuity of 

 the broad occiput with the trunk was uninterrupted by any cervical 

 constriction. The ribs commence at the second vertebra, but by 

 a bifurcate head ; and so continue, articulating with both par- and 

 di-apophyses until the confluence of those processes, when they 

 become single-headed. The ribs rapidly increase in length, which 

 is greatest at the middle of the thoracic-abdominal cavity, and 

 then gradually diminish to short and straight appendages, resem- 

 bling detached transverse processes, in the tail. The longer ribs 

 are grooved longitudinally ; their lower ends are united to hasm- 

 apophyses, subdivided into two or three overlapping slender 

 portions, the lowest articulating with a median transverse style, 

 pointed at each end, representing the haemal spine, and completing 

 the hasmal arch in the abdomen. In the tail the hasmapophyses 

 are simple, and attached by ligament, above to the centrum, and 

 below to one another. 



§ 20. Vertebral column of Sauropterygia. — In this extinct 

 order of acpiiatic Reptiles the vertebral bodies had their terminal 

 articular surfaces either flat or slightly concave, or with the 

 middle of such cavity a little convex. In certain genera the 

 neck-vertebras were uncommonly numerous ; this was remarkably 

 so in the Plesiosaurus, fig. 45, in which those vertebras consist of 

 centrum, neural arch, and pleurapophyses. The latter are wanting 

 in the first vertebra ; but both this and the second have the 

 hypapophyses. The cervical ribs are short, and expand at their 

 free end. They articulate by a simple head to a shallow pit, which 

 is rarely supported on a process, on the side of the centrum. 

 The body of the atlas articulates with a large hypapophysis 

 below, with the neurapophysis above, with the body of the axis 

 behind, and with part of the occipital condyle in front ; and all 

 the articulations, save the last, may become obliterated by 

 anchylosis. The hypapophysis forms the lower two-thirds, the 

 neurapophysis contributes the upper and lateral parts, and the 

 centrum forms the middle or bottom of the cup for the occipital 

 condyle. The second hypapophysis becomes ^confluent with the 

 inferior interspace between the bodies of the atlas and axis. 2 As 

 the cervical vertebras approach the dorsal, the costal pit gradually 



1 CLXV. 2 CLXVI. 



E 2 



