EXPLANATION OF PLATE XVIIL 



Rhinobatis columnac. 



Fig. I. — Dorsal view of a very young intra-uterine embryo; 

 the yolk-sac and the yolk-stalk are not shown, 

 X lA 

 ,, 2. — Dorsal view of an embryo of a much later stage 

 ,, 3. — Ventral view of the above ; the tail is not shown. 



,, 4. — Diagram of a specimen dissected from the ventral 

 side to show the relations of the yolk-stalk, etc. 

 X ii. 



,, 5. — Transverse section of the yolk-stalk as seen with the 

 low power of the microscope. 



,, 6. — A portion of the above highly magnified to show the 

 constitution of the wall. 



