External Structure of Paleozoic Crinotds. 457 
separately articulated and sometimes ankylosed with the outer 
margin of the plate above. This peculiarity not only exists 
in the arm-plates, but is conspicuous in the radials, thus pro- 
ducing apparently an articulate structure of the whole skeleton 
and indicating some degree of flexibility in the body as well 
as the arms. The interradial portions appear sometimes 
depressed, and in other cases swollen or bulged out, showing 
that they probably yielded to a moderate expansion or con- 
traction of the body-walls, due to the mobility of the radial 
parts, which likewise involves a flexibility of the summit. I 
have not been so fortunate to find the summit of any of these 
genera perfectly preserved; but I feel convinced from what I 
have observed that it did not consist of a soft skin. In Ony- 
chocrinus, the genus which possessed evidently the greatest 
expansive power, the radial plates are frequently found spread 
out horizontally, and J have found towards the inner or ven-. 
tral side of the radials rather large imbricating plates, to 
which smaller ones are attached which connect with the plates 
of the interradial series, and which decrease in thickness in- 
wardly. In several specimens I found the inner part or 
centre of the disk covered by a number of thin, very small 
plates, whose arrangement could not be made out; but it is 
highly probable, from their size and shape, that they formed a 
kind of scaly integument which was pliant and flexible, thus 
facilitating a contraction or expansion of the dorsal portions. 
The close relationship existing between Onychocrinus, 
Forbestocrinus, and Taxocrinus renders it almost certain that 
their summit was similarly constructed. In Ichthyocrinus the 
peculiarities in the radial portions are less strongly marked, 
and the genus has no interradial plate ; but as it agrees other- 
wise so nearly with Tazxocrinus that it is sometimes difficult 
to separate them, we may feel sure that this Silurian genus 
forms no exception to the general rule, but that its mouth 
was covered as in other Paleozoic Crinoids. 
That the summit in several genera has not been discovered 
is no proof that it consisted of soft material. During the 
eighteen years that I collected at Burlington I obtained 
several hundred of the most perfect specimens of Cyathocri- 
nus, some of them as perfect in most of their parts as if dredged 
from the ocean ; but only two specimens have been discovered 
in which the summit was preserved, and only a single Scaphio- 
ertnus. That this could happen at a locality where even the 
finest tissues of the most delicate internal organs are preserved, 
is somewhat astonishing; but yet it can be accounted for by 
the fact that the pieces which cover the central opening, as 
also the small alternating plates forming the ambulacral canal, 
Ann. & Mag. N. Hist. Ser. 5, Vol. i. 31 
