146 CATALOGUE OP THE BLASTOLDEA. 



CHAPTER VIII. 



DESCRIPTIONS OF THE SPECIES. 



The diagnoses of the numerous species of Blastoidea which are represented in the 

 National Collection may be conveniently preceded by the following table of the orders, 

 families, and genera. 



Class BLASTOIDEA. 



Order REG UL A RES, E. & C, 1886. 

 Pedunculate Blastoids with a symmetrical base, in which the radials and ambu- 

 lacra are all equal and similar. 



1st Family. PENTBEMITIDJE, d'Orbigny, 1852 (emend. E. & C., 1886). 

 Base usually convex and often much elongated. Spiracles five, but sometimes 

 more or less completely divided by a median septum. Their distal boundary formed 

 by side plates. Hydrospires concentrated at the lowest part of the radial sinus. 

 Pentremites, Say. Pentremitidea, d'Orb. Mesoblastus, E. & C. 



2nd Family. TEOOSTOJBLASTID^E, E. & C, 1886. 

 Ambulacra very narrow and descending sharply outwards from the much restricted 

 peristome. Deltoids usually limited to the summit and rarely visible externally. 

 Eancet-plate entirely covered by the side plates. Spiracles generally double, appear- 

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

 plates. 



Troostocrimis, Shumard. Metablastus, E. & C. 



Tricoelocrinus, Meek and Worthen. 



3rd Family. NUCLEOPLASTIIJ^E, E. & O, 1886. 

 Calyx usually globular or ovoidal, with flattened or concave base and linear ambu- 

 lacra. Spiracles distinctly double, and chiefly formed by the apposition of notches 

 in the lancet-plate and deltoids. 



(i.) Subfamily ELMACBJM'DM, E. &■ C, 1886. 

 Posterior deltoid divided into two parts by an anal plate. Anus distinct from 

 posterior spiracles. 



Elaacrimts, Roemer. 



