196 CATALOGUE OF THE BLASTOIDEA. 



much more robust species than T. Reinwardti, having larger, wider, and more 

 expanding ambulacra, with a more truncated and therefore more spacious summit. 



Locality and Horizon. Deccatur County, Tennessee: Niagara Group (=Wenlock 

 Series), Upper Silurian. 



Genus METABLASTUS, gen. nov. 



Troosticrinus , Shumard (pars), Trans. St. Louis Acad. Sci. 1865, vol. ii. no. 2, p. 384 (note). 

 Troostocrinus, Meek & Worthen (pars), Proc. Acad. Nat. Sci. Philad. 1868, p. 356. 

 Troostocrinus, Meek & Worthen (pars), Report Geol. Survey Illinois, 1873, vol. v. p. 507. 

 Beltmnocrinus ?, Munier-Chalinas (MS.), Journ. Coucliyliol. 1876, tome xvi. p. 105 (non 



White). 

 Belocrinus?, Munier-Chalmas (MS.), Bull. Soc. Geol. France, 1881, tome ix. p. 503. 

 Troostocrinus, E. & C. (pars), Ann. & Mag. Nat. Hist. 1882, vol. ix. p. 247. 



Gen. Char. Calyx narrow, slender, fusiform, or sub fusi form ; summit usually acu- 

 minate, always contracted, and sometimes subtruncate or a little convex ; base 

 elongate, sometimes tapering rapidly, sometimes truncate, triangular or triangular- 

 pyramidal, flattened below on all three sides ; section pentagonal above, triangular 

 below ; periphery nearly halfway from the summit. Radial plates long and narrow, 

 the limbs much shorter than the bodies ; sinuses narrow and deep. Deltoid plates 

 small and inconspicuous, confined to the immediate neighbourhood of the summit. 

 Ambulacra short, narrow, and usually sublinear, deeply set in the sinuses; side 

 plates, eighteen to sixty in number, quite concealing the lancet-plate. Spiracles ten 

 linear slits between the ambulacra and the deltoid ridges; the posterior pair separate 

 from the anus and nearer the peristome than it. Four hydrospire-folds on each side 

 of an ambulacrum. 



History. "We have been led to separate this genus from Troostocrinus partly in 

 consequence of Mr. Wachsmuth's discovery (which he generously communicated to 

 us) that the anal deltoid of T. Reinwardti is different from its fellows, and appears 

 externally ; and partly because we were struck by the fact that there is an anal 

 spiracle in this species as in T. Grosvenori, but not in any of the others which have 

 been hitherto referred to Troostocrinus (PI. III. figs. 14, 15; PI. XVIII. fig. 15). 

 All of these agree in having the two posterior spiracles separate from the anus and 

 opening nearer the peristome than it, just as in Tricoelocrinus (PL XIX. figs. 15, 16). 

 The combination of this character, with the presence of five similar deltoids and a 

 trihedral basal cup, appears to us to be of generic value. We were at first unwilling 

 to act on Mr. Wachsmuth's discovery and limit the genus Troostocrinus to T. Rein- 

 wardti, with the possible addition of T. Grosvenori, as we hoped that he might have 

 been able to discuss the question himself. But as he felt unable to take it up, in 

 consequence of his other engagements, he generously waived his own claims to 

 priority and left us free to act, as we have already explained on p. 112. With his 

 concurrence, therefore, we now propose the genus Mctnblastus with the characters 

 given above. 



