769 
of Edinburgh, Session 1871-72. 
total number of the arms, and sometimes one of the four arms 
given off from the brachial axillaries again divides, in which case 
the total number of arms is increased. The structure of the disk 
is much the same as in the species of the genus previously known. 
The Apiocrinid^: to which the remaining two fixed Crinoids 
must be referred, differ from all other sections of the order in the 
structure of the upper part of the stem. At a certain point consi- 
derably below the crown of arms the joints of the stem widen 
by the greater development of the calcified ring, the central cavity 
scarcely increasing in width. The widening of the stem-joint 
increases upwards until a pyriform body is produced, usually very 
elegant in form, in which one would suppose looking at the out- 
side that the viscera were lodged. It is, however, nothing more 
than a symmetrical thickening of the stem, and the body cavity 
occupies a shallow depression in the top of it inclosed within the 
plates of the cup ; the basals and radials are much thicker and 
more fully calcified than in other crinoids, but they are normally 
arranged. 
The stem is usually long and simple, until near the base, where 
it forms some means of attachment; either as in the celebrated 
pear encrinites of the forest-marble, a complicated arrangement of 
concentric layers of cement which fix it firmly to some foreign 
body ; or as in the chalk Bourguetticrinus and in the recent Bhizo- 
crinus , an irregular series of jointed branching cirri. 
The Apiocrinim: attained their maximum during the Jurassic 
period, where they are represented by numerous and fine species 
of the genera Apiocrinus and Millericrinus. The chalk genus 
Bourguetticrinus shows many symptoms of degeneracy. The head 
is small, and the arms are small and short. The arm joints are so 
minute that it is difficult to make up anything like a complete 
series from the separate fragments scattered through the chalk in 
the neighbourhood of a cluster of heads. The stem, on the other 
hand, is disproportionately large and long, and one is led to suspect 
that the animal was nourished chiefly by the general surface absorp- 
tion of organic matter, and that the head and special assimilative 
organs are principally concerned in the function of reproduction. 
The genus Rhizocrinus possesses all the essential characters of 
the family. 
