CRINOIDS. 
299 
are well adapted to retain their hold. The stem itself passes slowly from a rigid 
vertical attitude to a curved or even drooping position.” Other crinoids, like 
Herpetocrinus, have, however, a single fulcral ridge running across the surface of 
the joints, allowing far more play between the latter. In Platycrinus the joints 
had an elliptical outline, and the fulcral ridge formed the long diameter of the 
ellipse. Such a structure would naturally give the stem great power of bending, 
but only in one plane. This restriction was got over by giving the joints a slight 
skew, so that the stem was twisted like a corkscrew and capable of movement in 
every direction. In RMzocrinus the same end is attained by each joint being so 
twisted that the fulcral ridge at the top 
is at right angles to the one at the 
bottom. These types are, however, 
merely side branches from the main 
stem of crinoid evolution; the chief 
advance having proceeded along the 
lines of free locomotion. At various 
periods forms have existed, which, 
having once tasted liberty, have gradu¬ 
ally dropped all traces of their former 
attachment. Thus, Agassizocrinus of 
the Coal-Measures is a crinoid that has 
nothing left of its stem but a solid knob 
at the base of the cup; Millericrinus 
of the Great Oolite has been found at 
all ages and stages of development, the 
young individuals with a normal stem 
which gradually withers as the animal 
gets older, till in full-grown specimens 
it is a mere tapering process. Uinta- 
crinus and Marsupites from Cretaceous 
beds, are two genera as unlike in 
essential structure as crinoids well 
could be, but resembling one another in 
having thin-plated large cups, without 
the smallest relic of a stem. A little 
crinoid of Jurassic age called Thiolliericrinus appears, however, most nearly 
related to, if it be not the actual ancestor of, most of the free-moving unstalked living 
forms. It seems to have been related to Bourgueticrinus and Rhizocrinus, but, 
like Millericrinus, gradually dropped its stem, while the upper joint of the stem 
coalesced and began to bear cirri. In the common feather-star (Antedon) of the 
British seas this process has gone yet further; the animal breaks away from its 
stem when quite young, but retains the uppermost swollen and coalesced segments 
of the stem, which form one solid mass bearing a number of cirri, while the two 
lower circlets of cup-plates almost entirely disappear, so that only the upper circlet 
of plates, from which the arms arise, remains. The Antedonidce, which have all 
arisen since Jurassic times, include not only numerous species of Antedon, but at 
MEDUSA-HEAD PENTACBINID. 
a, Crown and part of stem (nat. size) ; b, Upper surface of 
the body, the arms broken away, showing the food- 
grooves passing to the central mouth. 
