Tin: AMi:i i.aci; \. 43 



by far the greater number of specimens collected, leaving for subsequent consideration 

 the covering-plates which roof in their median grooves (PI. I. fig. 8; PI. XV. fig. 12), 

 and the SO-called pinnules at their sides, which arc hut rarely found preserved (PI. III. 

 fig. 13; PI. XL figs. 10, 17). 



The ambulacra id' Pentremites, as usually exposed, are five broad and more <>i L< • 

 petaloid areas, which radiate outwards from the peristome towards the dorsal part of 

 the calyx (PI. 1. figs. 1-6 ; PI. II. figs. 1-7, 14-30). Each of them is marked 

 by a median groove, which continues the outward extension of the radial angles of 

 the mouth between the proximal ends of two adjacent interradial plates. Tins 

 groove, which in perfect specimens is roofed in by the covering-plates (PI. I. 

 fig. 8), is continued down the median line of the ambulacral held, being much more 

 marked in some forms than in others, and terminates against the body of the radial 

 which supports it (PL I. rig. 2 ; PL III. figs. 5, 7 ; PL XVI. fig. 22). It corresponds. 

 as we believe, to the food-grooves on the disc and arms of a Crinoid, and it may 

 therefore be termed the ambulacral groove. It occupies the median line of the broad 

 lancet-plate, as it was well called by Roemer. This fills up the central portion of the 

 ambulacral field, the space between its lateral edges and the sides of the radial sinus 

 being occupied by the side plates or pore-plates of lloemer (PL I. figs. 1-3, 5, 6 ; 

 PL XII. figs. 13, 14 ; PL XVI. fig. 21). The ambulacral field is thus traversed from 

 end to end by two lines of suture, which separate the median lancet-piece from the 

 side plates at its edges, as is well shown in PL I. figs. 3, 5, 8, 9. They are not always 

 visible, however, only appearing in specimens which have undergone a certain amount 

 of weathering, as pointed out by Roemer L The lateral edges of the lancet-piece are 

 very well seen in specimens from which the side plates have fallen away more or less 

 completely, as shown in PL I. fig. 5. This individual, and also the Pentremites 

 robustus represented on PL XVI. fig. 21, show very well the boundaries of the lancet- 

 plate at its proximal end, where it lies over the suture between two adjacent deltoids. 

 This poiut is still better illustrated, however, in the specimen of Pentremites piriformis 

 represented in PL I. figs. 6 & 7. The lancet-plate of one ambulacrum is intact, 

 though its proximal side plates have fallen away, thus increasing the size ot the spi- 

 racular openings ; while in the two other ambulacra the lancet-plate is altogether 

 removed, though its shape is indicated by the deep impression of its under surface m 

 the proximal ends of the deltoid pieces. On the left-hand side of fig. a fragment 

 of the lancet-plate has remained behind in the deepest part of this impression, 

 while on the right it has been entirely removed, as is shown more clearly in rig. > . 



The relations of the central end of the lancet-plate are, however, seen best of all in 

 another specimen of Pentremites piriformis, which we owe to the kindness ot Dr. G.J. 

 Hinde (PL XII. rigs. 13, 14). It is evident from this preparation that the under side 

 of the lancet-plate is by no means so simply constructed as its upper surface. The 



1 Archiv f. Naturgesch. L851, Jahig. ami. Bd. i. pp. 333, 



u 2 



