MKHRIA.M: TKIASSIC ECHTHYOSAUBIA. 



ill 



farther away from the caudal fin, interfere to a less extent with the movement 

 of the tail, and have a more definite and permanent value in the swimming 

 mechanism of the animal. It might therefore be considered at the outset as 

 a more difficult matter to determine affinities by the use of these elements than 

 by comparison of the bones of the pelvis. 



84 



85 



Fig. 84. Ichthyosaurus commuiiin Conybeare. Pectoral arcli, much ivduccil. .S'c, scapula; Co, cora- 



coid; Cl, clavicle; Id, interclavicle. (After Woodward.) 

 Fig. 85. Baptanodon discus Marsh. Pectoral arch, X l/iu- Sc, scapula; Co, coracoid; Cl, clavicle; 



Id., interclavicle; H, humerus. (Aftei Gilmore.) 



Probably, for the reasons given above, the anterior arch of the Triassic 

 ichthyosaurs does not show the degree of correspondence with the pectoral 

 girdle of early diaptosaurians that is generally evident in comparison of the 

 posterior girdle with that of these forms. There is really no one of the diapto- 

 saurian groups in which the correspondence is known to be as close in the 

 anterior arch as it is in the posterior one. In Stereosternum (fig. 87) the 

 anterior arch is not perfectly known, but the coracoid appears to be oval and 

 imperforate in the young, and of somewhat similar form but perforate in the 

 adult. The supposed scapulae of a young individual 24 are semi-lunar in form, 

 and the interclavicle shows a diamond-shaped expansion at the anterior end of 

 a long and slender shaft. In Palaeohatteria (fig. 86) the coracoids are small 

 and discoidal, the interclavicle is closely similar to that of Stereosternum, and 

 the scapula seems to differ mainly in the lack of excavation of the posterior 

 border. 



In comparison with the above forms the broadly triangular interclavicle of 

 Mixosaurus suggests the expanded anterior end of this element in Stereoster- 

 num and Palaeohatteria, though the differences are considerable. The scapula 

 of Stereosternum resembles that of Delphinosaurus, the broad perforated 

 coracoid is a little like the perforated coracoid of Cymbospondylus, and the 

 imperforate oval form corresponds to the type seen in Merriamia. There is no 



single ichthyosaurian form in which all of the characters of Stereosternum or 

 / 



" Osborn, H.F., Mem. Am. Mus., vol. I, p. 487. 



