MORPHOLOGICAL PART. 41 
stem is enlarged somewhat as in Apioerinus, except that the joints of the en- 
larged part in the Ichthyocrinide are very short, and increase but very 
slightly in length downward. These plates, which have no internodals inter- 
posed, extend to the full length of the inflated part, and their number varies 
in different species from about twenty to fifty, but is constant, or nearly so, 
in the same species. They are followed distally by a large prominent 
joint, from which the internodes begin, The latter are formed in the usual 
way, larger and smaller joints following each other at intervals, and the 
nodal joints are rather prominent. 
The stem of a Platycrinus has generally no internodes, and all young joints 
were introduced next to the basals. The joints are elliptic, and the apposed 
faces of the joints throughout this genus are provided with articular ridges, 
which follow their long diameters. A similar structure occurs in the recent 
Rhizoerinus and Bathycrinus, and both of them are apparently destitute of 
internodals. The Silurian Marsupiocrinus, however, with a circular stem, 
which is otherwise most closely allied to Platycrinus, always has well defined 
internodes; and this forms perhaps the best distinction between the two 
genera, 
The absence of internodals is not confined to specimens with elliptic 
stems, or to those with articular ridges. They are wanting also in Mespi- 
locrinus with a round stem, and in which the joints rapidly attain a length of 
from three to four times their diameter (Plate II., Fig. 3). In Rhodocrinus 
there is, so far as observed, but a single ossicle to each internode, and 
throughout the stem a larger plate alternates with a smaller one. 
In a few Palaeozoic Crinoids, the whole stem is divided longitudinally, its 
joints being either quinque- or tri-partite. The former is the case in Ohio- 
erinus, Ectenocrinus, Barycrinus, Anomalocrinus, and probably others; while a 
tri-partite stem has been observed only in Heferocrinus. The stem segments 
alternate with the proximal plates of the calyx; 7. ¢., they are interradial 
in dicyclic, and radial in monocylic Crinoids. 
Most Crinoids are provided with cirri, which are given off from the nodal 
joints at intervals, either throughout the whole length of the stem, or only at 
its distal end. The former is more generally the case among the later Cri- 
noids, while in the majority of Paleozoic forms the cirri are restricted to the 
lower part. In Neocrinoids they are more regularly distributed, and occur 
in whorls; in Paleeocrinoids they are generally arranged singly, and at irregu- 
lar intervals, The Pentacrinide have five cirri to each nodal joint, which 
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