148 C. W. Andrews — On the Pelvis of Cnjptoclidus. 



Another point worthy of notice is the extreme backward slope of 

 the ilia. This is so great, that if, when the isehia and puhes are 

 resting on a plane surface, a line he drawn along the long axis of 

 the ilium, it makes an angle of little more than 30° with that 

 surface. In many lizards and chelonians the ilia are directed 

 backwards to a considerable extent, but in none, as far as I am 

 aware, to such a degree as this. It seems not improbable that in 

 the living animal the ventral surface of the pelvis sloped downwards 

 and forwards, so that the position of the ilia would be less oblique 

 to the vertebral axis than might be supposed. 



The causes of the peculiar structure of the acetabulum and of the 



great backward slope of the ilium must, no doubt, be sought in 



the mechanical conditions under which the animal existed, and 



although these conditions are too little known and too complex to 



permit of a complete solution of the problem, the following tentative 



explanation may be offered. It is generally admitted that the 



Plesiosaurs are descended from land reptiles, in which, of course, 



one of the chief functions of the skeleton would be to support the 



soft parts, and in the case of the pectoral and pelvic girdles to 



transmit the weight of the body to the supporting limbs : in this 



case the thrust on the heads of the humerus and femur is mainly 



a vertical one. When such an animal becomes wholly aquatic in 



its habits these conditions are completely changed. In the first place, 



the body is supported on all sides by the pressure of the water ; in 



the second, the limbs no longer bear up the body weight, and if, 



as in the Plesiosaurs, they are used as paddles, there would be a 



nearly horizontal (forward) thrust of the heads of the humerus and 



femur at each stroke. In the case of the pelvic-girdle of the 



Plesiosaurs the result is exactly what, a priori, we should expect 



from such change of conditions. The upper end of the ilium is 



a fixed point (to the sacral vertebra?), about which the forward thrust 



. of the femur would tend to produce rotation leading to the backward 



slope we have seen actually exists. In the next place, the same 



force would produce the elongation from before backwards of 



. the acetabulum ; and this elongation, together with the backward 



rotation of the ilium, seems to have led to the separation of the 



latter bone from the pubis, and the result is an elongated acetabulum 



constituted by the articular surfaces of the pubis, ischium, and 



ilium, arranged in a linear series from before backwards. As to 



the means by which this has been brought about, opinions will 



differ, but in any case it seems at least highly probable that the 



change in the conditions of life has resulted in the peculiarities above 



. described. In this connection it is interesting to notice that among 



the Chelonia, certain of which show greater similarity to the 



Plesiosaurs in their pelvic-girdles than any other reptiles, the marine 



, forms, such as Chelone, have backwardly directed ilia, while the 



exclusively terrestrial ones, such as Testudo pa.rda.lis, have those 



bones nearly vertical ; in the fresh-water tortoises both types occur. 





