108 CATALOGUE OF THE BLASTOIDEA. 



The spiracles are very inconspicuous, and have the same general relation to the 

 ambulacra as those of Schizoblastus ; but the manner in which the deltoids are 

 grooved for the reception of the hydrospire-cauals rather suggests that the type is 

 more closely allied to Granatocrinus. 



The mode of formation of the spiracles in Acentrotreniit.es, Cryptoblastus, Elasa- 

 crinus, Schizoblastus, and Mctahlastus is not very different from that which presents 

 itself in Orophocrinus. The two first genera resemble Mesoblastus and Granatocrinus 

 in having an hydrospire-plate coming up between the lancet-plate and the wall of the 

 radial sinus (PL VII. fig. 15; PI. XIII. tig. 19) ; but this is altogether absent in 

 the other types, as in Pentremites. 



The position of the spiracles of Accntrotremit.es is very much the same as in Oropho- 

 crinus stcUiformis, viz., commencing just in front of the radio-deltoid suture, and 

 therefore some little way from the mouth (PI. XL figs. 8, 9; PL XIII. fig. 19); 

 but the causes of this are not quite the same in the two genera. In the Oropho- 

 crinus, the proximal thirds of the deltoid plates approach one another very closely 

 and the ambulacia lie upon their apposed edges (PL XI. fig. 9). Some little way 

 from the mouth, however, the sides of the deltoid plates diverge rapidly until they 

 almost meet the radio-deltoid sutures, so that the plates have an elongately hexa- 

 gonal form. The radial sinus of Orophocrinus stcUiformis does not therefore come 

 so close up to the peristome as it does in most other Blastoids, Granatocrinus being, 

 perhaps, the type which approaches it most nearly in this respect. The position of 

 the lancet-plate in G. Norwoodi is well seen in the cast shown on PL VII. fig. 7, 

 which may be usefully compared with the worn specimen of Orophocrinus stelliformis 

 represented in PL XL fig. 9. In this individual the pointed proximal end of the 

 lancet-plate comes directly into contact with the deltoids, and this condition is 

 continued outwards while the width of the lancet-plate increases ; but when it begins 

 to diminish again, the plate comes away from the deltoid, and a cleft, the spiracular 

 opening, appears between the two, being continued across the radio-deltoid suture, 

 and down the side of the ambulacrum until the lancet-plate, or rather, the under 

 lancet-plate meets the radials and closes it up (PL XL figs. 8, 9 ; PL XV. fig. 13). 



In Acentrotrcmites, on the other hand, so far as can be judged from the ambula- 

 crum figured on PL XIII. fig. 19, the lancet-plate comes close up to the peristome, 

 just as in Cryptoblastus melo (PL VII. fig. 15), and is completely concealed by the 

 side-plates of the ambulacrum. These gradually become broader and broader till 

 they reach a point just inside the radio-deltoid suture, so that they fill up the rapidly 

 increasing gap between the ambulacral edges of the deltoids. At this point, however, 

 the edge of each deltoid is slightly notched, and as the side-plates do not extend into 

 the notch, a gap is left between them and the deltoid, which is, in fact, the spiracle 

 (PL XIII. fig. 19). Between this opening and the peristome there are no marginal 

 pores to the ambulacrum ; but on its distal side there seems in our only specimen to 



