No. 444] P TER A SPID/E A ND CEP HA LA SPfDsE. 845 



shows a well defined opening corresponding in position and out- 

 line with the mass of trabecule below. In some specimens, the 

 opening is covered by a special group of loose, irregular polygonal 

 plates, well shown in PI. I. Fig. 3. 



The mass of trabecular below the post-orbital valley is conspic- 

 uous and is often very sharply outlined (PI. I. Fig. 2). It is 

 oblong, pointed in front, and broad and somewhat angular behind. 



The post-orbital valley when seen from above, in well pre- 

 served specimens, presents a clearly defined oblong opening, 

 similar in shape to the underlying mass of trabecular, and filled 

 with polygonal plates similar to those of the marginal openings. 



In Thyestis, Fig. 9, I have found marginal and post-orbital 



