REPORT ON THE CRINOIDEA. 341 



(itself a syzygy) in a singular specimen of Pentacrinus decoriis, whicli was clredgccl by 

 the " Blake" (ante, p. 336). 



In Metacrinus, however, the number of primitive radials is typically either five or 

 eight. In both cases the second and third of these eventually become united by syzygy, 

 and the latter, which corresponds exactly to the third radial in the abnormal Pentacrinus 

 miilleri already mentioned (PI. XV. fig. 2), bears a pinnule, as does also the next joint, 

 which is sometimes followed by the axillary (PI. XXXIX. fig. 1 ; PI. XLIII. fig. 2). 

 When there are eight primitive radials the original fifth and sixth joints become united 

 by syzygy in the same way ; so that the ray is described as consisting of six joints, of 

 which the second and fourth are syzygial, both of these as well as the third and fifth 

 bearing pinnules (PI. XLVI. ; PI. L. fig. 1). 



These pinnules also afibrd a very good distinguishing character of Metacrinus. Their 

 lower and middle joints are very large and massive, having a considerable vertical height 

 in addition to their thickness from side to side ; while one or two of the basal joints are 

 nearly cubical in appearance (PL XXXIX. fig. 1 ; PL XLIII. figs. 2, 4; PL XLVL; PL 

 XLVIII. fig. 1 ; PL L. fig. 1). The later joints, while remaining long and high, gradually 

 diminish in breadth so that they come to be prismatic, with a sharp dorsal edge. The 

 distichal and palmar pinnules have the same characters as those on the radials, though 

 in a less prominent degree. 



The shape and large size of their joints are well shown in PL XXXIX. fig. 2, and in PL 

 XLIII. fig. 2. There is no regularity as to the side of the ray on whicli the first pinnule 

 occurs ; so that the same interradial space may be occupied by the first pinnules of two 

 rays as is shown on the right of PL XXXIX. fig 1 ; while on the left of the same figure 

 the second pinnules of two rays also occur in the same interradius. This not unfrequently 

 causes a certain amount of crowding ; and it is not uncommon for one of the later radial 

 or lower distichal pinnules to be very much flattened laterally and almost knife-like in 

 appearance, while the corresponding pinnules on the other rays of the same individual 

 are large and massive. 



About the level of the distichal axillary, or rather farther out if the number of arms 

 be large, the stout lower pinnules begin to lose their distinguishing characters and to 

 pass gradually into the ordinary pinnules of the free arms. The transition is very 

 difficult to explain without a long series of figures illustrating its successive stages, and it 

 varies a good deal in difierent types; but it may be generally described as a flattening of 

 the pinnule-joints (as it were) against the arm and against the succeeding pinnules. 

 They thus retain a considerable height in a vertical (dorsoventral) direction and have a 

 sharp dorsal edge, with the exception of the lowest joints which are set in a different 

 plane from their successors and are often of a much greater width. This is the case, for 

 example, in the palmar and lower lirachial pinnules of Metacrinus moseleyi as seen on 

 the left hand ray of the specimen represented in PL XLVI. ; while in Metacrimis 



