PHORMOSOMA PLACENTA. 31 



rows along the outer edge of the interambulacral plates, or the plates in 

 specimens of the same size may carry the same vertical rows nearlj to the 

 apical system, but placed in the central part. 



This arrangement, together with the greater thickness of the test, gives to 

 these specimens a very differenl facies, closely resembling in the arrangi 

 men! of the tubercles P. rigida and other small Echinothuriae, which, when 

 writing the Report on the Echini of the Challenger, 1 could noi refer to any 

 of the species collected by thai expedition. 



The color of the specimens with a thick tesi is of a deep claret, while 

 usually the majority of the specimens of P. placenta with a thinner tesi are 

 of a brick color, or of a yellowish orange, darker on the actinal side, and 

 near the ambitus on the abactinal side, and becoming quite lightrcolored, 

 either pinkish or yellowish, inwards the apical system. 



In the youngest specimen of Phormosoma collected by the Blake, 

 measuring 8 nun. in diameter, the plates of the actinal system have 

 already assumed the characteristic arrangement of the adult, although there 

 are as vel bul three rows of imbricating plates. We do noi find in this 

 species, as in two of the species of the Challenger collection | /'. tenuis and 

 P.urarm ' Challenger Echini, PL XVIH". Figs. 7, 12), thai the buccal ten- 

 tacular plates differ in size from those following them, recalling the general 

 arrangemenl of the buccal plates of young Echinidae proper. On the con- 

 trary, although the Blake specimens were no larger than the correspond- 

 ing stages of the Challenger species, they show that in this species of 

 Phormosoma the characteristic structural features of the Echinothuriae are 

 already developed. This, as [ have already shown, we also find to be the 



case in young Cidaridae. It thus appears thai in s e of the species of 



Echinothuriae the Echinid features of the actinostome seem to be retained 

 much longer than in others, and do no1 even seem always i" be developed, 

 if we may judge from the young stages of the Wesl Indian Phormosoma 

 here described. 



The mode of formation of the coronal plates, and of the actinal and 

 abactinal systems, is well shown in the Beries of the young of Phormosoma 

 I have had occasion to examine. As 1 have suggested before, the Echino- 

 bhurise are the mosl embryonic of the Echini. 'The coronal plates when 

 they first appear, somewhat indistinctly defined, in young specimens measur- 

 ing about 8 mm. in diameter, show very clearly thai there is no special line 

 of demarkation to be drawn in the young Btages between the abactinal and 



