PLATE XV. 
1, 2. Astropyga sp. 
Fig. 1. Abactiuul system of young Astropyga 13 mm. iii diameter. 
Fig. 2. Actinostome of same. 
3-19. Phormosoma placenta Wrv. Thoms. 
Fig. 3. Young Phormosoma 8 mm. in diameter, showing the abactinal system, and a part of the test. 
Fig. 4. Somewhat younger specimen 8 mm. in diameter, seen from the actinal side, showing the 
sphaeridia, suckers, and pedicellariEe of that side of the test. 
Somewhat older Phormosoma 17 mm. in diameter, seen from the abactinal side of tjje test. 
The same, seen from the actinal side. 
Older specimen 20 mm. in diameter, seen from the actinal side. 
The same, seen from the abactinal side. 
Older specimen 28 mm. in chameter, seen from the abactinal side. 
The same, seen from the actinal side. 
Phormosoma 41 mm. in diameter, seen from the abactinal side. 
The same, seen from the actinal side. 
Fig. 13. Portion of anal system of young Phormosoma, 22 mm. in diameter, showing the mode of 
deposition of the limestone meshes forming the anal plates. 
Figs. 14-19 show the gradual passage of mere accumulations of limestone cells, as in Fig. 14, which 
indicate the first position of the future tuljercles, to Figs. 16, 15, 17, 18, and 19, in which 
the scrobicular ring, the mammary boss, and the perforation of the future tubercles are 
little by little indicated. 
Fig. 
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