an almost perfect Skeleton of the Plesiosaurus. 385 



are removed from their place, and are scattered over the mass of shale in which 

 they are imbedded. In consequence of this accident, we are admitted to a full 

 view of the ribs and sterno-costal arcs and pelvis which remain undisturbed. 

 Fourteen large ribs may be counted, and twenty-one dorsal or lumbar vertebrae 

 appear dispersed, though their exact original number cannot be ascertained. 

 The last of these lies over the pubis, and has, close to it, a short false rib. 



Twenty-three caudal vertebrae are remaining; and as about 3 of the extreme 

 ones appear to be wanting, we may probably assume this part at about 26 joints, 

 the whole vertebral column then will number about 90 joints, viz. 35 cervi- 

 cal, 6 anterior-dorsal, 21 dorsal and lumbar, 2 sacral, and about 26 caudal. 

 The proportions of the whole of these parts will stand nearly thus : taking the 

 head as I, the neck will be as 5, the body as 4, and the tail as 3, the total 

 length being, as was before remarked, 13 times that of the head. 



The chevron bones beneath the tail are finely exhibited ; but this part, hav- 

 ing been fully described in my former papers, suggests no new remarks, 

 excepting that its shortness must have prevented its being used, as in fishes, 

 as an instrument of impulsion in a forward direction, and that it was therefore 

 probably employed only as a rudder to steer the animal by horizontal flexure, 

 or by a sudden vertical stroke to elevate or depress it while swimming through 

 the water. 



The anterior sternal portion is greatly concealed by the vertebrae and ribs 

 lying over it : these might be carefully removed and replaced, and the struc- 

 ture of this important part ascertained. From several imperfect specimens 

 which I have examined, it appears to have been complicated in its structure, 

 and nearly to have resembled that of the tupinambis. 



The posterior part of the sternum consi-sts of a central bony arc, crescent- 

 shaped, and swelling in the middle ; to its horns are applied two sterno-costal 

 branches, which appear as usual to have been connected with the extremities 

 of the ribs by cartilages : the nice adaptation of these parts is beautifully dis- 

 played in the specimen. 



The pelvis is finely displayed, and resembles the usual type of this part in 

 reptiles, of which the turtle perhaps affords the best example for comparison 

 with the fossil : the ilium is reduced to a long and slender bone, which might, 

 if seen detached, be mistaken for the os pubis ; that of several species of turtle 

 is exactly similar. The ischium is like that of most reptiles ; and the pubis, as 

 is also common in this class, is so greatly dilated as to be liable to be mistaken for 

 the ilium if found separately. All these parts are very nearly in situ, and the 

 manner in which they unite to form the acetabular socket is easily perceived ; 

 the oval formation between the ischium and pubis is also quite distinct. 



3d2 



