PLATE XII. 



Fig. 1. Portion of the disk of a young Porpita, measuring over ^y in diameter, seen from the upper side. 



Fig. 2. Same seen from the lower side. 



Fig. 3. Portion of the disk of a young Porpita, measuring ^" in diameter. 



Fig. 4. Young Porpita, measuring f^" in diameter, in which a portion of the disk of the vascular system 

 has remained simple. Seen from above. 



Fig. .5. Portion of the disk of a young Porpita, nearly in stage of Fig. 4, seen from above, but the vascu- 

 lar system is branching. 



Fig. (i. Portion of the disk of a young Porpita at about the same stage with simple vascular system, but 

 proportionally longer tubes. 



Fig. 7. Portion of the disk of a vuung Porpita about in stage of Fig. 6, seen from the lower side of the 

 disk. 



Fig. 8. Portion of the disk of a young Porpita somewhat more advanced, the vascular system more 

 branching. 



Figs. 9, 9'. Portion of the disk of a young Porpita measuring about \" in diameter. The ramifications 

 of the vascular system are limited to the margin of the mantle. 



Fig. 1(1. Portion of the disk of a young Porpita measuring /j" in diameter. 



Fig. 11. Portion of the disk of a Porpita measuring not quite \" in diameter. 



Fig. 12. Marginal folds of the edge of the mantle. 



Fig. 13. Portion of the disk of a Porpita measuring 1" in diameter, showing the system of anastomos- 

 ing tubes of the mantle. 



Fig. 14. A portion of the same, showing the star-like irregular termination of this anastomosing system. 



Fig. 15. Magnified view of a part of the white plate. 



