EXPLANATION OF PLATES 207 



Fig. 



13 Lateral view of same, x 2 



14 Brachial valve, x 2 



15 Internal cast of pedicle valve showing casts of muscular and 



vascular impreesions. x 2 



16 Interior view of pedicle valve showing muscle impressions 



and vascular trunks, x 2 



17 Interior view of brachial valve, x 2 



18 Cardinal process and crural plates, x 2 



19 Broken shell showing the geniculate character of the posterior 



and lateral margins, x 2 



20 Enlargement of surface, x 5 

 Pebbles of group 5 



