BLASTOIDEA. 



463 



first time. The other genera most largely represented in the 

 Carboniferous rocks are Granatocrinns, Schizoblastus (fig. 332, c), 

 Codaster (fig. 331, a), Orofthocrinus (fig. 331, b), and Mesoblastas. 

 Above the horizon of the Carboniferous Limestone no remains of 

 Blastoids have been as yet discovered. 



As regards their classification, the Blastoids are divided by Mr 

 Etheridge, jun., and Dr P. H. Carpenter, into the two primary 

 groups or orders of the Regulares and Irregulares, the former 

 including pedunculate types with a symmetrical base and having 

 equal and similar ambulacra, while the latter comprises sessile forms 



Fig. 332. — a, Pentremites pyriformis, viewed sideways, showing a portion of the column ; 

 b, Summit of the calyx of Pentremites cervinus, showing the ambulacral areas and the apical 

 apertures ; c, Side-view of Schizoblastus melonoides ; d, Summit of Schizoblastus neglectus. (Figs. 

 a and b are of the natural size ; c and d are slightly enlarged.) (After Hall, and Meek and 

 Worthen.) 



in which the base is usually unsymmetrical, and one ambulacrum 

 and the corresponding radial are different from their fellows. The 

 following table exhibits the families adopted by the authorities just 

 named, with the genera included in each : — 



Order I. Regulares. 



Family 1. PentremitidcE. — Base usually convex and often much elon- 

 gated. Spiracles five, but sometimes more or less completely divided by 

 a median septum. Their distal boundary formed by side-plates. Hy- 

 drospires concentrated at the lowest part of the radial sinus. The genera 

 Pentremites (fig. 328), P entremitidea, and Mesoblastus are included in 

 this family. 



Family 2. Troostoblastido2. — 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 generally double, appearing as 

 linear slits at the sides of the deltoid ridge, but not bounded distally by 

 side-plates. The genera Troostocrinus, Metablastus, and Tricoelocri?tus 

 belong here. 



Family 3. Nucleoblastidoz. — Calyx usually globular or ovoidal, with 

 flattened or concave base and linear ambulacra. Spiracles distinctly 

 double, and chiefly formed by the apposition of notches in the lancet- 



