THE AMBDLAOBA. 55 



sponding to the ambulacra! or food-groove of a Crinoid, and we shall therefore speak 



of it by this name. The five grooves radiate outwards from the mouth just like the 

 disc-ambulacra of a Crinoid, and we suppose that there can be little doubt of their 



having been ciliated during life. As in the Crinoid, too, the food-groove of a Blastoid 

 maybe covered in and thus converted into a tunnel, though the covering is but rarely 

 preserved in the fossil state (PI. I. fig. 8 ; PL 111. figs. 2, 3 ; PI. XV. figs. 11, 12). 

 The mouth, or more strictly the peristome (for the actual mouth was doubtless 

 membranous, as in the Urchins), is generally more or less distinctly stellate in outline, 

 the short rays of the star being the gaps between the pointed central ends of the 

 deltoid plates (PI. I. figs. 4-G ; PI. 111. figs. 1, 14 ; PI. IV. figs. 1, 17 ; PI. V. figs. 10, 

 19 ; PI. VI. figs. 7, 8, 13 ; PI. VII. figs. 5, 14, 15 ; PI. IX. figs. 5, 8, 14, 15 ; PI. XII. 

 figs. 1, 4). It may be simply pentagonal (PI. IV. fig. 14 ; PI. V. fig. 2), or even round, 

 as in Phcenoschisma nobile, P. acutum (PI. XI. fig. 3; PL XIV. fig. 11), and in 

 Eleutheroerinus (PI. XIX. fig. 6), while in some cases it is extremely constricted 

 (PI. IV. figs. 8, 10 ; PI. V. figs. 4, 10, 23 ; PI. XI. fig. 9). 



In all cases alike, however, each radiating space between the proximal ends of two 

 deltoids is continued as a slight groove on to the surface of the lancet-plate, which 

 lies flush with the interdeltoid sutures, and is continued outwards to its distal extre- 

 mity, as is well seen in Pentremites (PI. I. figs. 4-0), Cryptublastus melo (PL VII. 

 figs. 14, 15), and Orophocrinus (PL XL fig. 9). The proximal ends of the deltoids are 

 usually bordered by a delicate crenulation which extends along each side of the inter- 

 deltoid suture, and then down the sides of the food-groove on the lancet-plate (PL IV. 

 figs. 1,8; PL VII. figs. 10, 11, 14,15; PL VIII. fig. 2 ; PL IX. figs. 5, 8, 14 ; PL XIII. 

 fig. 14). 



In the broad ambulacra of a Pentremites such as P. pyriformis, with its lancet- 

 plate completely visible, the food-groove gives off lateral branches alternately on 

 opposite sides; and well-preserved specimens show that each of these branches, like 

 the food-groove itself, has its double border of crenulation which extends outwards 

 towards the edge of the lancet-plate and then gradually disappears (PL I. figs. 1, 2). 

 The very extensive development of this crenulation upon the broad ambulacra of 

 Pentremites seems to have led Hambaeh to assert that they were covered by an elastic 

 plated integument, but his account of it will best be considered when the various 

 forms of the side plates have been described. 



In the narrow linear ambulacra of Codaster, Mcsoblastus, Granatocrintts, and 

 Cryptoblastus, however, almost the whole of the lancet-plate is concealed by the side 

 plates which rest upon it, and the crenulation of its surface is limited to the median 

 groove (PL IV. fig. 4 ; PL VII. figs. 14, 15 ; PL VIII. fL-. 15 ; PL IX. fig. 10 ; PL \. 

 figs. 8, 11, 12; PL XIII. fig. 14) ; but in other forms of Mcsoblastus, in Acentrotremites, 

 Orophocrinus, Pentremitidea, and in I'liamoschisma, the lancet-plate is entirely con- 

 cealed by the side plates, which meet one another above its median line, so that no 



