BLASTOIDEA. 



459 



A, c). This median groove clearly corresponds with the "food- 

 groove " of the arm and disc of a Crinoid, and may therefore be 

 termed the "ambulacral groove." The ambulacral groove is ex- 

 cavated in the middle line of a long pointed plate (fig. 329, b, /) 

 which runs down the centre of the ambulacral field and was termed 

 by F. Roemer the " lancet-plate." The two halves of the lancet- 

 plate, in well-preserved specimens, are seen to be crossed by numer- 



a/i 



Fig. 328. — A, and b, Side-view and base oiPentremites pyriformis: b b, Basals ; dd, Radials ; 

 p p, Ambulacral fields, c, Summit of Granatocrinus Norwoodi (after Meek and Worthen), en- 

 larged, showing the plated membrane covering the mouth (;«), the spiracles (sj>), the anus {an), 

 and the summits of rhe ambulacral fields (a;;/), d, Summit of Pentremites sulcatus (after Ferd. 

 Roemer), showing the mouth denuded of its covering-plates and placed in the centre of the 

 spiracles (sp) : an, Anal opening ; am, One of the ambulacral fields. From the Carboniferous 

 rocks. 



ous transverse grooves, which open centrally into the main ambula- 

 cral groove. As the lancet-piece is narrower than the ambulacral 

 field, there exists on each side of the former an interval, separating 

 the sides of the lancet-plate from the forks of the bounding radial 

 on each side. This interval is occupied on each side by a row of 

 small quadrilateral ossicles or "side-plates" (fig. 329, s), which 

 complete the ambulacral field superficially. Each ambulacral area 

 is thus traversed from end to end by two lines of suture (fig. 329, 

 b), which separate the median lancet-plate from the side-plates at 

 its edges. 



