EXPLANATION OF PLATE XVIIL 



I'ig. I. xlnibonyckia 'i poststriaia, Eth. fil. Kight valve exbibitinj^ tbe 

 elevated uuibone, posterior radiations, and concentric laminae. 



„ 2. Anterior end of the same Bpecinien showing the large Vjyssal sinus 



[The figures, from drawings by Mr. G. H. Barrow, of the Australian 

 Museum, are of the natural size.] 



