BRITISH FOSSIL REPTILES. 119 



The vertebral column corresponds in its general slenderness 

 with the characteristic form of the head. The number of con- 

 stituent vertebrae appears to be at least as great as in any pre- 

 viously described species of Ichthyosaurus : they are more 

 variable in the antero-posterior direction, and have a more 

 rounded and less angular contour than in ?i\\j other species : 

 their anterior and posterior articular surfaces are simply con- 

 cave. A greater proportion of the terminal caudal vertebrse 

 present the laterally compressed form than in other Ichthyo- 

 sauri, which would indicate 1-hat the tegumentary caudal fin was 

 of greater relative extent. The atlas or odontoid epiphysis of 

 the vertebra dentata separates more easily from the body of that 

 vertebra than is usual in other species of Ichthyosauri; and 

 there appear to be only two subvertebral bones, viz. that beneath 

 the occipital condyle, which represents the atrophied body of 

 the atlas ; and the second, developed in the angle between the 

 axis and odontoid epiphysis ; the third, which is usually situated 

 between the axis and third cervical vertebra, is wanting in the 

 present species. This simplification of the apparatus for fixing 

 the neck accords with the light and slender character of the 

 head. The vertebrae gradually increase in thickness, or antero- 

 posterior extent, as they approximate the caudal region, whence 

 they gradually diminish in all their dimensions. But at the 

 posterior part of the abdomen, and beginning of the tail, they 

 are relatively thicker in the direction of the axis of the body than 

 in the other species of Ichthyosaurus. I count fifty vertebrae 

 between the atlas and the first caudal vertebra. 



The costal tubercle in the caudal vertebrae is situated near the 

 anterior part of the centrum. The ribs are long and slender, 

 and appear, in the present sp'ecies, gradually to increase in 

 length to near the posterior end of the vertebral column, and 

 then to shorten more abruptly than usual. 



The extremities of the Ich. tenuirostris are characterized in 

 the first place by a disparity in the size of the fore and hind 

 pairs similar to that which obtains in the Ich. communis and 

 Ich. intermedins, and the fore paddles are more particularly 

 distinguished by their massive proportions, as compared with 

 the vertebrae. 



The clavicles are slender, slightly expanded at the middle, and 

 contracted at the two extremities ; their anterior margin is in- 

 flected about 1| inch from their sternal extremity. The sca- 

 pulae are relatively larger than in the preceding species, bent 

 and expanded at their humeral extremity. 



The coracoids have a broad neck, a slight inferior emargina- 

 tion, and a deep and narrow superior notch. 



In a well-preserved specimen of Ich. tenuirostris in the Bir- 

 mingham museum, 



