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in denuded specimens, in which the two plates can also be seen 

 to be separated at the mouth end and to leave a rather spacious 

 cavity between, which is covered over with a membrane in 

 alcoholic specimens. Each mouth-plate bears only one crest which 

 runs along its whole length and is double, there being a shallow 

 but distinct furrow on it (PL II, fig. 17). Corresponding to this 

 double structure of the crest, there are two parallel series of 

 spines on each plate. The spines of the inner series, i.e. the one 

 on the side of the mouth-plate turned towards its fellow, are 

 rather short but robust and rounded at the end ; the one at the 

 mouth end is larger than the others but only slightly longer ; there 

 are in all some ten or eleven spines in this series. The spines of 

 the outer series are considerably more slender than those of the 

 inner and are usually closely appressed to the latter ; the spine 

 at the mouth end is, however, nearly as large as that of the 

 inner series, so that there are in each interradius four spines of 

 large size at the mouth end (PI. II, fig. 18). The outer series 

 may contain from eight or nine to some twelve spines, and may 

 extend the whole length of the mouth-plate or may terminate at 

 about two-thirds of its length. 



Ventrolaterals. — The ventrolaterals are less numerous than 

 in P. misaldcnsU, but extend outwards over two-thirds of the 

 length of the arms (PL II, fig. IG). From the character and 

 arrangement of the ventrolaterals of the arms we may say that 

 the innermost series extends farthest into the arms, and that the 

 outermost series terminates at the third or fourth inferomarginal. 

 Again, if we adopt for the sake of comparison, the same limit to 

 the interradial disk area as in P. misaJdensis, there are in the 

 present species some 30-32 plates in each interradius. The ventre- 



