Striictare of the Shoulder Girdle in Lystrosaui'us . 141 



The precoracoid in general resembles that bone in Oudenodon. 

 It articulates with the coracoid, the sternum, and the scapula. It 

 is a moderately thin, flat bone. Its outer side is very deeply notched 

 — the notch forming with the scapula a large precoracoidal foramen. 

 The precoracoid probably forms no part of the glenoid cavity. 



The coracoid is not well preserved in either of the specimens. In 

 the first specimen only a small part of the inner border is preserved. 

 In the other specimen, however, the greater part of the bone is seen, 

 but it appears to be considerably distorted, so that I have thought it 

 unwise to figure it. The outer and anterior parts of the bone probably 

 agree fairly closely with those parts in the coracoid of Oudenodon, 

 but the inner and posterior half of the bone differs considerably owing 

 to its being twisted to articulate with the posterior part of the outer 

 side of the sternum instead of with the anterior and outer side 

 as in Oudenodon. 



The scapula bears a close resemblance to that bone in Oudenodon 

 and Dicynodon. The upper part of the blade is broad and moderately 

 flat from side to side. A little below the middle of the bone there is 

 in front a well-developed acromion process. It may be described as 

 a triangular plate which on passing forwards is directed slightly 

 inwards from the plane of the general surface of the scapula. The 

 lower part of the scapula is considerably broader than in Oudeno- 

 don. The posterior part of the lower end forms probably half of 

 the large glenoid cavity. The anterior expansion of the lower end 

 passes further forward than the acromion and forms a long articula- 

 tion with the precoracoid. It is notched to form part of the 

 precoracoidal foramen. 



The shoulder girdle in Lj/strosaurus is interesting as a specialised 

 variety of the Anomodont type. Among both mammals and reptiles 

 we find instances of the remarkable modifications in the shoulder 

 girdle that may follow the adoption of a purely aquatic life. Thus in 

 the Pythonomorpha the clavicles and interclavicle become quite rudi- 

 mentary or entirely lost, and in the Whales the clavicles disappear. 

 In the Plesiosaurians both the clavicles and interclavicle become 

 greatly reduced and the interclavicle may disappear. In the Ichthyo- 

 saurians, though the clavicles are moderately well developed, the 

 interclavicle is usually small. 



It would therefore seem probable that the reduction of the inter- 

 clavicles, the increase in size of the sternum, and the associated 

 shifting backwards of the coracoid and precoracoid have been 

 brought about by the aquatic habits of Lystrosaurus. 



