a ‘ 
REPORT ON THE CRINOIDEA. 155 
In the recent Pentacrinidee, however, there are never as many as two hundred arm- 
joints, and several of the outermost are entirely devoid of functional pinnules, having 
nothing but mere stumps in their place, without any ambulacral plates at the edges of 
their ventral grooves (Pls. XXXVIIL, XL., XLIT—XLIV., XLVIII., XLIX.). Nearly 
one-third of the length of the arm of a Pentacrinus or Metacrinus may be in this un- 
developed condition, which is never met with among the Comatule. 
In almost all Neocrinoids the articular facets on the first radials occupy the whole 
width of their distal faces, so that the lowest parts of the rays, whether divided or not, 
are of nearly the same width as the radial plates which bear them. In many Palo- 
crinoids, however, such as Platycrinus, the articular facet of the first radial simply 
occupies the middle of its distal edge, so that the lowest parts of the rays are quite small 
compared with the calyx. This is the case among the Neocrinoids in Hyocrinus 
(Pl. VL), Plicatocrinus, and to a less extent also in Marsupites; while it is very 
characteristic of the young stages of the Pentacrinoid larva of Comatula. But the 
occurrence of this feature is far more general among the Palzocrinoids than in the late: 
forms. 
Except in some species of Hnerinus, the arms of a Neocrinoid are invariably uniserial, 
i.e., composed of a single series of joints which are placed end to end, and bear pinnules 
alternately on opposite sides. The arms of the earlier Palsocrinoids were also composed 
of single joints; but in all the three principal divisions of the group the composition of 
the arms changed from a single to a double row in the Upper Silurian period. If this is 
to be considered as an advance in development, then all the Post-Triassic Crinoids are in 
this respect permanent larval forms. According to Wachsmuth and Springer’s descrip- 
tion of Mariacrinus, the double jot arrangement is brought about by the coalescence of 
two contiguous arms, an approach to which may perhaps be found in the flattening of 
the sides of the lower parts of the arms in many Pentacrinidz and Comatule, and in 
Holopus (PI. Va, fig. 3; Pl. XV. fig. 2; Pl. XVI. fig. 1; Pl. XXX. fig. 1). But this is 
merely superficial, and the alternate arrangement of the pinnules is unchanged, which is 
far from being the case in Paleocrinoids with biserial arms. Other Palzocrinoids, such 
as the Ichthyocrinidee and some Cyathocrinide, seem to have had no pinnules at all, 
though the arms branched freely. 
It will be apparent from what has been said above that except in one or two points, 
e.g., the symmetry of the calyx, the differences between the so-called Tessellata and 
Articulata are not so great as has been sometimes imagined. But there is one other 
structural character of great importance, to which attention has been especially drawn of 
late by Messrs. Wachsmuth and Springer as distinguishing the two groups. I refer to the 
condition of the mouth and of the oral surface generally. 
The American paleontologists! define the cup of a Palewocrinoid as “closed on the 
1 Revision, part i. p. 30. 
