REPORT ON THE CRINOIDEA. 77 



represented in PI. L. fig. 2. It seems at first sight to be perfectly bare, but careful 

 examination proves it to be covered by very closely set small plates with ill defined 

 boundaries. 



The ambulacra of Pentacrinus u'ljvilh-thomsoni form rather prominent ridges, which 

 are composed of four irregular rows of plates. The plates of the two inner rows arc 

 somewhat elongated transversely and generally closed down over the grooves, representing 

 the covering plates of the pinnule ambulacra. 



A well plated disk also occurs in Pentacrinus alternicirrus (PI. XXVI. figs. 1,2); 

 but the ambulacra are less heavily plated than in Pentacrimis wyville-tJwmsoni, and are 

 therefore not so readily distinguished from the anambulacral plates. These are 

 sometimes larger than in the Atlantic species, and are pierced by abundant water-pores 

 which are not shown in the figure ; but they do not always fit quite closely together, so 

 that gaps of bare perisome are visible here and there. As in Pentacrinus wyville- 

 thomsoni, the plates are generally larger in the anal interradius than elsewhere. 



A disk of Pentacrinus naresianus was drawn for Sir Wyville Thomson by Mr. Black 

 (PI. XXX. fig. 2) ; but it seems to have been mislaid or else cut into sections, for it has 

 not come into my hands. So far as can be judged from the figure, the anambulacral 

 plates were small ; while the ambulacra appear to be well-defined ridges and to come 

 into close union around a very small peristome, which is thus entirely concealed by the 

 apposition of their large covering plates. 



I have not seen a disk either of Pentacrinus maclearanus or of Pentacrinus hlakei ; 

 but in Pentacrinus miilleri and Pentacrinus decorus it is far from being as completely 

 plated as in the species already noticed (PI. XVII. fig. 10 ; PI. XXXIV. fig. 2). For the 

 anambulacral plates are generally isolated and not in contact with their fellows. They 

 are small and numerous in Pentacrinus miilleri; but in Pentacrimis decorus they are 

 fewer in number and comparatively large, some of them containing as many as twenty 

 water-pores.^ 



There are about four irregular rows of plates on the ambulacra, the inner ones 

 being elongated, and sometimes standing up rather prominently at the edges of the 

 grooves. 



The relation of the food-groove to the arm-joints varies greatly in the difierent species 

 of Pentacrinus, so that mere fragments of the arms can be identified by the characters 

 of their ambulacra, quite apart from any peculiarities of their arm- and pinnule- 

 joints. 



The middle line of the upper surface on each joint of the brachial skeleton is occupied 



l)y a groove of variable depth and width, to which MiiUer gave the name " ai-m-groove " 



(PI. XVII. figs. 1, 4, 7, 8, 9). It is bordered on each side by the more or less prominent 



muscle plates of the successive joints ; and the vascular structures which are partially 



' These are omitted in the figure, and the plates are dra\\Ti too close together. 



