THE BLASTOIDEA 



single. The anus opens through the posterior spiracle, i.e. through the 

 oral end of post. A ; the position is less primitive than in Codaster, but the 

 greater size of post. A seems primitive. Metablastus, Eth. & Carp. (1886), 

 is represented by doubtful species in the Silurian of N. America, less 

 doubtful from Devonian of Europe, and undoubted from Lower Carboni- 

 ferous of N. America. It closely resembles Troostocrinus in form ; but 

 the A are all equal, small, and confined to the summit, while the two 



Fio. VIII. 



Troostocrinus Eelnwardtl. 1, section across a radius, much enlarged. 2, upper part of 

 theca, posterior view. 3, oral surface of theca. Lettering as before. As-\-sp, anus and con- 

 fluent spiracles. (2 and 3 slightly altered from Etheridge & Carpenter.) 



posterior spiracles are distinct from the anus. In structure the ambulacra 

 scarcely differ from those of Troostocrinus; but the lancet-plate of 

 M. lineatus is said to have three longitudinal canals '.*, the meaning 

 of which is not obvious (cf. Schizobkistus). The base is elongate, and 

 triangular in section, with flattened sides. Tricoelocrinus, Meek & Worthen 

 (1868), Carboniferous, N. America, and (2) Queensland (Fig. XV. 2), owes 

 its name to three excavations along the interbasal sutures, corresponding to 

 the three flattened sides of Metablastus. A feature of more importance is 

 the enclosure of the distal portion of the hydrospires within the thickness 

 of the radial plate ; this is probably due to the upward growth of the 

 floor of the sinus, as has taken place, independently and to a less extent, 

 in Pentremites. Eleutherocrinus, Shumard & Yandell (1856), Devonian, 

 N. America, is probably a descendant of Troostocrinus. It has, however, 

 lost its stem and adopted some mode of life that has affected its symmetry. 

 The place of the stem is occupied by the small r. ant. B. The two other 

 BB are pushed a little to the side and produced up the theca to meet the 

 broadened and shortened 1. post. R. The remaining RR, with their ambu- 

 lacra, are much like those of Troostocrinus and Metablastus ; this 1. post. 

 R, however, bears a short ambulacrum with only seven side -plates on 

 either side, but these greatly widened transversely to the median groove, and 

 curved distalwards (Fig. IX.). There are small A, and (apud Shumard) 

 spiracles at their oral ends on either side of each normal ambulacrum. 

 The anus is stated by Wachsmuth (in Whiteaves, 1889) to lie on the 

 right upper margin of the abnormal R, and therefore at the aboral end 

 of the A. Sections across the ambulacra (Eth. & Carp., 1886) show a 

 lancet-plate with large longitudinal canal, and seven hydrospire-folds, 

 arranged as in Troostocrinus, on either side of each ambulacrum in its 



