E. Billings on the structure of Crinoidea, Oystidea, etc. 285 
above Actinocrinus in rank, and between it and the adult Star- 
fish. In none of its stages does it resemble a Bipinnaria either 
in form or in structure. 
9. On some o - die come that have been advanced against the 
advocated in the preceding notes. 
In all the wed species of the existing Echinodermata, the 
mouth is situated in the center of the ambulacral system, and 
it is contended that this fact proves boy such must have been 
its position also in the paleozoic form 
This reasoning is not strictly logit. It is true that in the 
known existing species, the mouth is in the center, but it 
does not certainly follow that it is so in all the Echinodermata, 
living and extinct. Whether it be so or not in any particu- 
lar fossil species whose structure may be under investigation, 
question of fact which can only be positively determined by 
direct observation of specimens. On appealing to these we find 
that, in a large proportion of the foss i forms, there is no aper- 
ture in the perisome at the ambulacral center. It also becomes 
evident by the comparison that, in general, one paleozoic ad 
cies resemble the embryonic stages of some of the recen 
Kehi eet and that in these, (Bipinnaria for grants mn 
mouth is interradial. Rules such as is relied o case 
afford a ae amount of presumptive vient which how- 
ever, cannot prevail against material and visible fac When 
we e r 
center is situated, it is pices useless to pag one by de- 
duction.* 
as to induce the belief that it lived parasitically on the ntances 
The only answer I can make to this is that, admitting the facts, 
we must Bre = that space was left for a stream of water to 
Devonian and. ©. Carboniferous ages, were carfivorous, i i 
tg ved by the bored shells and Crinoids that are occasionally » 
ound. I have seen a number of such specimens, and several 
* The position of the ambulacral center may thus be found. When the mouth 
i ambulacral tubes usually converge to the center of the vault. 
easily recognized by its possessing a greater number of plates than do an one of 
the other four interradii. On the opposite side of the fossil is the sayrde arm. 
The ambulacral center is always situated between this arm ai and the mouth, never 
on the side of the mouth toward the azygos aacealia. 
