PLATE 1 



EXPLANATION OF FIGURES 



1 Median portion of pectoral girdle of Hexanchus. Note the medial 'inter- 

 cepted cartilage' which bears all the relations of a presternum in higher forms. 

 The dotted lines indicate how this girdle may be transformed into that of the 

 Amphibia with pre- and post-omosternum. Drawing from a dissection made 

 by the author in the U. S. National Museum. 



2 Pectoral girdle of Acanthias vulgaris. Note that all the parts of the 

 girdle of higher forms are present, including a sternum. Compare with figure 

 35 of the fetal marsupial. 



Cr, coracoid Sc, scapula 



POSt, pre-omosternum SSc, suprascapula 



PSt, presternum St, sternum 

 PtOSt, post-omosternum 



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