Till: HTDEOSPIEES AM) M'l I; \< I.Ks. Ill 



the ambulacrum ; whereas in Orophocrinua QO pores are left at all. For in the English 

 and American .species of the genus the distal portion of the ambulacrum is in 

 immediate and continuous contact with the radials (PI. XI. fig. 9 ; PL XV. Hlts. 1-0, 

 11). 



The character of that portion of the deltoid plate which intervenes between 

 the two spiracles in each interradius of the Nucleoblastidse or Troostoblastida? varies 

 considerably. In Acentrotremites the spiracles are separated by the whole width of 

 the deltoid (PI. XIII. figs. 18, 1'.)). In Elceacrintts and in Schizoblastus Sayi the 

 proximal end of the plate between the spiracles is tolerably broad and flattened or 

 even slightly concave (PI. III. figs. 1-3; PI. XVIII. tig. 16). In Cnjptoblastus mil,, 

 it is represented by a wide ridge (PI. VII. figs. 14, 15), which is still more reduced 

 in Schizoblastus melonoides (PI. VI. fig. 16), and smaller yet in the two Irish species 

 of this genus (PI. VIII. fig. 9 ; PI. XVI. fig. 12). The second of these (S. Bailii) 

 presents a transition towards Mcsohlastus, in which, however, the side plates 

 project beyond the edge of the lancet-plate and rest on the hydrospire-plate, so as 

 actually to cover in the canal beneath it, as shown in PI. VI. fig. 13. There are 

 also other links between the type of spiracles of the Xucleoblastidoe and Troosto- 

 blastidse, and that of the Pentremitidje. The deltoids of Metablastus li neat us were 

 figured by Shumard * as visible in a side view of the calyx. But we believe this to 

 have been an error, as in all the examples of this type which we have seen they do 

 not appear in this position at all (PL III. figs. 14, 15). Their outer ends are over- 

 lapped by the radial limbs just as in the Tricalocrinus figured on PL XIX. figs. 13, 

 15 ; and all that is visible of them in the summit is their proximal ends, each with a 

 strongly marked oral ridge rising from its centre. This ridge separates the two slit- 

 like spiracles just as the corresponding ridge does in Schizoblastus Bailii (PL XVI. 

 fig. 12) or in some forms of Pcntrcmitcs Godoni and P. elongatus (PL 1. figs. 5, 11), 

 though in the latter genus the side plates form the distal border of the spiracle, 

 which is not the case in the Troostoblastidae. In Triccelocrinus Woodmani this 

 ridge is only visible in the posterior interradius (PL XIX. figs. 15, 16), and the 

 spiracles of the other four interradii have very much the same relation to the deltoid 

 plates as those of some species of Pentremitidea (PL VI. figs. 16, 19) ; though the 

 two genera differ altogether in the relation of the spiracles to the side plates of the 

 ambulacra. 



The spiracles of Troostocrinus Rcinwardti are essentially like those of Triccelo- 

 crinus, except that the posterior pair are confluent with the anus, while the 

 anal deltoid is much larger than its fellows, and extends down on to the side of 

 the calyx. Eoemer 2 has given a good ngure of the summit of this type, in 

 which, however, the deltoids are erroneously represented as appearing externally in 



1 Trans. St. Louis Acad. Sci. IS.jS, vol. i. no. 2, pi. ix. fig. 3a. 

 • Archiv f. Xaturgesch. 1851, Jahrg. svii. BJ. i. Taf. iii. fig. 12 b. 



