86 THE BLASTOIDEA 



overspreading ambulacrum, but the admedian folds become gradually 

 lowered into the thecal cavity and open into a common canal (e.g. P. 

 clavata) ; thus each system of folds forms a " pendent" hydrospire-sac. 

 The second change is the slight notching or bevelling of the ends of the 

 side-plates and the projection of the outer side-plates so as to touch the 

 wall of the radial sinus ; thus is formed along each side of the ambulacrum 

 a series of openings by which water is admitted to the hydrospires, which 

 it bathes and then passes out again by the spiracles (e.g. P. lusitanica) ; 

 these openings are called " pores," but are not comparable to the haplo- 

 pores or diplopores of the Cystidea, to the water-pores of Crinoidea, or to 

 the ambulacral pores of Echinoidea. Pentremites, Say (1820, originally 

 Pentremite), Carboniferous, N. America (Fig. XV. 1), appears to be a 

 descendant of the American species of Pcntremitidea. The chief difference 

 is that the side-plates do not cover the lancet-plate, but rest against its 

 edge ; thus the ambulacrum becomes much broader and assumes a petaloid 

 shape (Fig. VII. 3, 4). A sub-lancet is developed, as in Orophocrinus ; 

 hydrospire-folds 3 to 9 according to the species, freely pendent (Fig. VII. 5), 

 as in some Pentremitidea ; but in some species the floor of the radial sinus 

 meets below the hydrospires at the distal end of the ambulacrum. A 

 canal runs between the hydrospires and the side-plates, emerging through 

 spiracles which may be single or double ; there is an anal spiracle, as in 

 Pentremitidea. Though limited in geographical and geological distribution, 

 Pentremites has more species and individuals than has any other Blastoid 

 genus; for this reason, and because it was the first of its class to be 

 introduced to science, it has usually been treated as the type of the Blas- 

 toidea. Zoologically considered, however, it merely represents the acme 

 of one particular line which thereafter died out. 



We return, therefore, to the Silurian, and take up the beginning of 

 another line of development, apparently connected in origin with that 

 which has just been traced and not very divergent therefrom. 



Troostocrinus, Shumard (1866, emend. Eth. & Carp., 1886), Silurian, 

 N. America, might be expected from its geological position to be a primi- 

 tive form ; and that it is such is shown by the structure of the ambulacra 

 (Fig. VIII.). The chief differentiation from the Codaster type lies in the 

 elevation of the radial processes ; the restriction of all A, except post. A, 

 to a very small truncate summit, so that the hydrospire-folds are almost 

 entirely formed out of the radial stereorn ; the narrowing of the radial 

 sinus, so that (as in some species of Phaenoschisma) the side-plates are 

 pushed up on to the top of the lancet-plate, while the hydrospire-folds 

 are pushed beneath it The outer side-plates are small, subtriangular, 

 squeezed out to the edge of the side-plates, with which they alternate ; 

 they touch the wall of the radial sinus so that "pores" are formed 

 between them. The hydrospire-folds are midway between the Codasterid 

 type and the pendent type ; their admedian walls, which support the 

 lancet-plate, are thickened, and tend to form " hydrospire plates," covering 

 the immediately adjacent folds. The canal that runs along above the 

 hydrospires and below the lancet and outer side -plates comes to the 

 surface at the oral end through a spiracle bounded by the A and lancet- 

 plate, and since the deltoid crests are slight, the spiracles are almost 



