REPORT ON THE CRINOIDEA. 155 



In the recent Pentacrinidse, 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 (Pis. XXXVIIL, XL., XLII.-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 mth among the Comatulse. 



In almost aU 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 Palseo- 

 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 

 (PI. VI.), 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 Palseocrinoids than in the later 

 forms. 



Except in some species of Encrmus, the arms of a Neoerinoid 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 PalsBocrinoids 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 joint 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 Pentacrinidse and Comatulse, and in 

 Holopus (PI. Va, fig. 3 ; PL XV. fig. 2 ; PI. 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 Palseocrinoids with biserial arms. Other Palseocrinoids, such 

 as the Ichthyocrinidse and some Cyathocrinidse, 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 diflferences between the so-called TesseUata 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 palaeontologists^ define the cup of a Palseocrinoid as " closed on the 



1 Revision, part i. p. 30. 



