THE BONJJS IN THE ENALIOSAUBIA. 301 



The pectoral girdle of birds is not like that of Ichthyosaurs ; 

 and the difference is largely due to the development of a ster- 

 num in birds. The sternum of a young struthious bird, while 

 its two halves remain separate, has quite the aspect of a pair of 

 potential coracoid bones. And with such a view the interpre- 

 tation of the keel in carinate birds as the potential interclavicle 

 would be in harmony, since it overlaps the line of union of the 

 two bones as in Ichthyosaurus. 



The compressed elongated scapula of the bird, enlarging at the 

 articular end, differs from that oi Ichthyosaurus more in its slender 

 proportion than in its plan, though it has in many water-birds an 

 acromial tubercle for the end of the clavicle, and does not receive 

 that bone along its whole anterior margin. 



The clavicle of the bird differs from the typical single clavicle 

 of Ichthyosaurus only in wanting connnexion with the margin 

 of the scapula and with an interclavicle (unless it is supposed to 

 occur when the clavicle articulates with the sternal keel). The 

 coracoids of birds differ from those of Ichthyosaurs in their elon- 

 gated form and in not meeting each other mesially. 



The pelvis of a bird is entirely unlike that of an Ichthyosaur. 

 In the Emu the pubis and ischium are more slender than in most 

 Ithyosaurs ; but the pubis has not the straight anterior margin 

 of Ichthyosaurus, and the ischium has a tubercle towards the 

 proximal end (by which, it meets the side of the pubis), which in 

 Ichthyosaurus is not developed. The ilium is totally different. 



In the limbs of birds there is no structural resemblance, either 

 in the forms of the bones or in their arrangement. 



§ 3. The Crocodilian Characters o/" Ichthyosaurus. 

 The crocodilian head is usually more depressed than in any Ich- 

 thyosaur, and, except in the G-avials, has not so pointed a snout, 

 while the surface of the cranial bones is always more or less pitted. 

 The chief changes necessary to convert the crocodile into Ichthyo- 

 saurios would be an enormous enlargement of the eye, so as to 

 raise it from its nearly flat position to a nearly vertical one. This 

 would draw the maxillary bone up till it was neai-ly vertical, 

 draw the prefrontal and postfroutal together above the orbit, 

 and allow an enormous median triangular space for the nasal 

 bones to expand in and encroach upon the frontal. The en- 

 larging of the orbits would enlarge the temporal fossae and ex- 

 tend the squamosals backwai'd. The vacant space in the Crooo- 



