PHOEMOSOMA 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 nearly 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 different facies, closely resembling in the arrange- 
ment of the tubercles P. rh/ula and other small Echinothuriae, which, when 
writing the Report on the Echini of the Challenger, I could not refer to any 
of the species collected by that expedition. 
The color of the specimens with a thick test is of a deep claret, while 
usually the majority of the specimens oi P. placenta with a thinner test 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 light-colored, 
either pinkish or yellowish, towards the apical system. 
In the youngest specimen of Phormosoma collected by the Blake, 
measuring 8 mm. in diameter, the plates of the actinal system have 
already assumed the characteristic arrangement of the adult, although there 
are as yet but three rows of imbricating plates. We do not find in this 
species, as in two of the species of the Challenger collection (P. tenuis and 
P. uranm ? Challenger Echini, PI. XVIIP. Figs. 7, 12), that the buccal ten- 
tacular plates differ in size from those following them, recalling the general 
arrangement of the buccal plates of young EchinidiB proper. On the con- 
trary, although the Blake specimens were no larger than the correspond- 
ing stages of the Challenger species, they show tliat in this species of 
Phormosoma the cliaracteristic structural features of the Echinothuria3 are 
already developed. This, as I have already shown, we also find to be the 
case in young Cidaridte. It thus appears that in some of the species of 
Echinothuriae the Echinid features of the actinostome seem to be retained 
much longer than in others, and do not even seem always to be developed, 
if we may judge from the young stages of the West Indian Phormosoma 
here described. 
The mode of formation of the coronal plates, and of the actinal and 
abactinal systems, is well shown in the series of the young of Phormosoma 
I have had occasion to examine. As I have suggested before, the Echino- 
thuriae are the most embryonic of the Echini. The coronal plates when 
they first appear, somewhat indistinctly defined, in j'oung specimens measur- 
ing about 8 mm. in diameter, show very clearly that there is no special line 
of demarkation to be drawn in the young stages between the abactinal and 
