E. Billings on the structure of Orinoidea, Cystidea, ete. 227 
of the fossil, and do not come in contact with the forked plates. 
The apex x of Pentremites is identical with the actinal center of 
Sea-urchins and Star-fishes, in which the mouth is situated. 
is here that the ambulacra originate and grow outward by the 
addition of new plates to their distal extremities. There can be 
little doubt that such was the mode of growth of the ambulacra 
of the Pentremites. The smaller extremity, therefore, of their 
ambulacra, which is received into the forked plate, is not the 
base, but corresponds with the apex of the ambulacrum of a 
Sea-urchin or of aStar-fish. It also represents the tip of the arm 
of a Crinoid. If the forked plate is radial, then the arrangement 
of the ambulacrum must be the same as that which would be 
exhibited in a Crinoid, = the upper end of the arm down- 
ward, and resting on the first radial, whilst the lower end 
would be upward, the sip leg formed of the second radial. 
From this it follows that the forked a do not belong to 
the radial, but to the perisomatic syst 
The five deltoid plates alternate With ¢ the forked plates, and 
are also perisomatic. 
t is not certain that the lancet omen st represent any 0 
those “plates which in the Crinoidea y called “ra- 
dials.” They are so arranged that if faye were loosened from 
the walls of ee cup, and their smaller extremities turned up- 
ward, whilst their bases or larger ends retained their posi- 
te ‘they would stand in a circle around the apex, as do the 
s of an ordina rinol Their bases would alternate 
with the apices of the deltoid plates. They would form the 
outside of the arms, whilst the grooves and pinnulz would be 
inside. Hach would bear, on its outer or destin’ aspect, two 
elongated sacks, the two hydrospires that ined to the am- 
bulacrum. I believe that the small groove in the ambula- 
crum of Pentremites was occupied by the ovarian tube only. 
If this be true, and if, also, the lancet plates represent the 
radial plates of the arms of the Orinoid, then the arm of 
Pentremites would have the respiratory portion of the ambu- 
print —_ on its dorsal, and the ovarian portion on its ven- 
tra 
In the we Crinoids, both the respiratory and ovarian tubes 
are a in the groove in the ventral side of the arm.* In 
as Say, who was the first to recognize the Blastoidea as a group dis- 
tinct f frou he Crinoidea, also supposed the function of the ambulacra to be 
rocesses; these may al orm 
Scatandaes in ent fa food to the mouth, which was, perhaps, provided with an 
i bosci e animal fe 
minute beings that ‘oe nded in the sea water, ~ navel it obtained them in the 
manner of the ape by taking them in wi The residuum of di- 
gestion appears to have been rejected through the mont (Jour. Acad. N. 8. 
Phil., vol. iv, 296, 1825). 
