150 THE VOYAGE OF H.M.S. CHALLENGER. 
of the body, they are often connected by a pavement of minute irregular plates, which 
may commence as low down as the level of the second radials, and are thought by 
Wachsmuth and Springer to indicate the presence of a flexible perisome. This is 
especially the case in some species of Taxocrinus, the rays of which must have been at 
least as free as those of many Pentacrinide and Comatulz, and much more so than those 
of Apiocrinus and Guettardicrinus. In these two genera, as in Uintacrinus, the calyx 
interradials are not only present but well developed, so as to increase the size and com- 
plexity of the cup. In fact the rays of Guettardicrinus are immovably united as far as 
the second brachial, either directly, or by the intervention of interradial plates ; while 
some species of Apiocrinus (Apiocrinus parkinsoni) have the second and third radials in 
close lateral contact with their fellows. Other species, however, with the arms free from 
the radial axillaries, have a well defined pavement of interradial plates, the lowest of which 
are large and regular and rest on the upper angles of the first radials, as in Apiocrinus 
rowsyanus ;* while the upper ones are smaller and more irregular, and pass gradually 
upwards into those of the ventral side. The same is the case in Marsupites. 
Many Pentacrinide and Comatulee have wide rays which are in close lateral contact 
just as in Apnocrinus parkinson (Pl. XV. fig. 2; Pls. XVIIL, XIX; Pl XXV-_; 
Pls. XX VIII.-XXX.), while others have the rays more separated from one another, but 
united by flexible perisome in which the joints of the lower pinnules and numerous small 
irregular plates are imbedded (Pl. XIII. fig, 1; Pl. XXXI; Pl. XXXVII. fig. 1; 
Pl. XXXIX. fig. 1; Pl. XLIX. figs. 1,2; Pl. L. fig. 1). These may cease at the level 
of the third axillaries, or pass up into the plating of the ventral side as in Apiocrinus 
roissyanus, Marsupites, and the Liassic species of Hxtracrinus. But they are never so large 
as in these fossils, and more nearly resemble the small irregular plating of the Ichthyocrinide. 
Thus then there are many Neocrinoids with no interradial plates in the calyx; and 
when these plates are present and well defined, as in Apiocrinus, Guettardicrinus, and 
Marsupites, or Uintacrinus, they are not limited to any special side of the calyx, but 
are equally distributed all round it; so that there is no distinction of the anal side, 
Thaumatocrinus of course excepted. 
In the Paleocrinoids, however, the pentamerous symmetry of the calyx is almost 
always disturbed by a greater or less modification of the plates on the anal side. The 
difference may be very slight, as in Phimocrinus and Cupressocrinus, which have the 
anal opening separating the muscle-plates of two adjacent radials. But even this 
character appears to be absent in the remarkable genus Hrisocrinus from the Upper 
Carboniferous of America, which has a calyx unusually like that of Knerinus; and also in 
Stemmatocrinus from the Russian Carboniferous, which is still more like Encrinus in the 
structure of the arms. 
Some forms have a special anal plate between two of the primary radials. This is the 
1 See woodcut, fig. 9 on p. 183. 
