190 CATALOGUE OF THE BLASTOIDEA. 



as compared with the other species known to us. The base is protuberant as in 

 M. crenulatus and M. Sowerbii (PL VI. figs. 9, 12, 14). 



Localities and Horizon. Clitheroe, Lancashire: Carboniferous Limestone (presented 

 by the late J. Rofe, Esq., F.G.S.). Yorkshire : Carboniferous Limestone (Coll. 

 Owens College, Manchester). 



Family TROOSTOBLASTIBM, E. & C, 1886. 



Definition. Ambulacra very narrow, and descending sharply outwards from the 

 much restricted peristome. Deltoids usually limited to the summit, and rarely 

 visible externally. Lancet-plate entirely covered by the side-plates. Spiracles gene- 

 rally double, appearing as linear slits at the sides of the deltoid ridge, but not bounded 

 distally by side plates. 



Remarks. We have established this family to include the generic types which 

 were first described as Pentremites Reinwardti, Troost, P. lineatus, Shumard, and 

 P. Woodmani, Meek & Worthen, and are referred by us to Troostocrinus, Meta- 

 blastus, and Triccelocrinus respectively (PI. III. figs. 14, 15; PL XII. fig. 11; and 

 PL XIX. figs. 13-16). They differ from the Pentremitidse in having a much 

 contracted summit and very narrow ambulacra, owing to the lancet-plate being 

 completely covered by the side plates, which do not bridge over the hydrospire- 

 canals, or form the distal border of the spiracles. The radials are always very 

 long, and the base is equally so in Troostocrinus and Metablastus, though not in 

 Triccelocrinus (PL XVI. fig. 18 ; PL XVIII. fig. 13;. The length of the radial 

 sinus varies a good deal, being shortest in Troostocrinus Reinwardti (PL XII. 

 fig. 11), but, except in the anal interradius of this species (Fig. VII. on p. 193), 

 the radial limbs extend right up to the summit and overlap the deltoids, so that 

 they do not appear externally (PL III. figs. 14, 15 ; PI. XII. fig. 11 ; PI. XIX. 

 figs. 13, 15). 



The combination of all these characters marks off the Troostoblastidae pretty 

 distinctly from the other families of the class. The one which they approach 

 most nearly is, of course, the Pentremitidse, some forms both of Pentremites and of 

 Pentremitidea having a very considerable resemblance to Troostocrinus and Meta- 

 blastus, as will be pointed out subsequently ; but a careful examination of sufficiently 

 well-preserved material reveals important structural differences. 



The geological history of the Troostoblastidae is a curious one. Troostocrinus 

 Eeinwardti occurs in the Upper Silurian of America together with some other types 

 which we cannot properly recognize, as they are only represented by internal casts 

 or broken specimens. No Devonian species are known in America, although the 

 family is represented in rocks of this age in England, France, and Spain ; while the 

 numerous Carboniferous species are all confined to America. 



The following scheme shows the mutual relations of the three genera belonging to 

 this family : — 



