cidarim;. 



17 



ambulacral and interambulacral areas is well shown on comparing the 

 number of the ambulacral plates which are opposite the actinal suture 

 of the fifth plate. In a specimen of 8 mm. (PI. 10, fig. 4) it is the tenth 

 ambulacral plate which faces the suture, and the fourteenth which faces 

 the upper suture. In a specimen of 10 mm. it is the fourteenth and the 

 nineteenth which face the sutures. In a specimen of 18 mm. it is the 

 twentieth and the twenty-sixth. In one of 21 mm. it is the twenty-second 

 and the thirtieth (PI. 10, fig. 7), and in one of 35 mm. it is the twenty-fourth 

 and the thirty-first (PI. 10, fig. 9). The action of the muscular fibres 

 attached to the scrobicular circle, in destroying the mamelon and mam- 

 mary boss of the secondaries, is already indicated in a specimen of 18 mm., 

 Fig. 40. In the earlier stages (PI. 10, fig. 4) the scrobicular circle of the 

 secondaries is not indicated. It becomes still better limited in specimens of 



Figs. 43-45. Dorocidabis panamensis. 



21 mm. (PI. 10, fig. 7) and of 35 mm. (PI. 10, fig. 9). The crenulation of 

 the fifth plate is only to be seen in a specimen of 35 mm. (PI. 10, fig. 9) ; 

 it is not developed hi earlier stages of growth. 



In the fifth plate of a specimen of 21 mm., the area on the two sides 

 of the primary tubercle is nearly the same ; the upper suture is but little 

 shorter than the lower (PI. 10, fig. 7). In the next stage (PI. 10, fig. 9) 

 the two sutures are of nearly equal length, the mamelon and its mammary 

 boss are in the centre of the plate, miliaries and smaller secondaries have 

 developed outside of the row of the secondaries edging the scrobicular 

 circle (PI. 10, fig. 9). 



It is quite feasible to follow the order of appearance of the miliaries 

 and secondaries. For instance, we can readily trace the order in which 



