124 Prof. H. G. Seeley. The Shoulder Girdle 



anterior to the coracoids, meeting in the median line, should rather be 

 precoracoids than scapulae in such Sauropterygia as show these charac- 

 ters. It has already been shown to be probable that the foramen 

 anterior to the coracoid is the precoracoid foramen, having under- 

 gone such an enlargement in transition from Nothosaurus to Plesio- 

 murus as does the obturator foramen between the pubis and ischium 

 in transition from the pelvis of Dicynodon to the pelvis of a Mammal. 

 Therefore the precoracoids may have ceased to be differentiated, even 

 as separate cartilages, and the coracoids may have grown forward at 

 the expense of this cartilage, just as the scapulas extended inward and 

 backward at its expense ; so that, while the scapulae are conveniently 

 so named, it may be recognised that in Elasmosaurus, Colymbosaurus, 

 Murcenosaurus, and their allies, the parts of the bone which meet in 

 the median line, and are in median contact with the clavicular arch, 

 are theoretically in the position of precoracoid elements, which con- 

 nect the scapulae with the coracoids. But since the Plesiosauridae 

 show no such median union of scapular elements, or ossifications in 

 front of the coracoids, it follows that there is no evidence that the 

 precoracoid was ossified at all, while the cartilage representing it, if 

 present, must have been a slender bar, comparable to the suggested 

 precoracoid cartilage in Ichthyosaurus, as shown by the absence of a 

 thick cartilaginous truncation of the anterior median termination of 

 the coracoids in Plesiosaurus. 



In the Anomodontia the plan of development of the shoulder girdle 

 has been modified by the great extension of the clavicular arch out- 

 ward and upward, so that the scapulae are rather on the type of the 

 Ichthyosauria than of the Sauropterygia. But the position and 

 relations of the Anomodont precoracoid furnish some support to the 

 interpretation given to the element in Ichthyosauria and Sauro- 

 pterygia; because, if the precoracoid foramen in Anomodonts were 

 theoretically enlarged to the dimensions seen in Colymbosaurus, Ple- 

 siosaurus, or Lariosaurus, it would be manifest that for so long as it 

 connected the scapula and coracoid it was Elasmosaurian; so long as 

 it remained attached to the extremity of the scapula only it would be 

 Plesiosaurian ; and so long as a remnant remained of cartilage in 

 contact with the inner border of the clavicle the condition would be 

 Lariosaurian. 



There is thus a fundamental difference of plan between the imper- 

 forate coracoid of Sauromorpha and the perforate coracoid of Ornitho- 

 morpha, which depends upon the way in which the precoracoid bone 

 loses its individuality. 



§ 3. Nomenclature of the Bones in the Clavicular Arch. 



Early writers regarded the median bone anterior to the coracoids in 

 Plesiosaurus as the sternum. Sir R. Owen named it episternum. Pro- 



