Bibliographical Notices. 415 



limited to the radial and iiiterradial plates ; their slits are parallel to, 

 and more or less completely concealed by, the ambulacra, often opening 

 externally through pores at their sides, and also by five or ten open- 

 ings round the peristome. Neither hydrospires nor ambulacra extend 

 below the basiradial suture. 



" Peristome naturally concealed by a vault of small plates, which 

 rarely exhibit any definite arrangement, and are continuous with 

 the covering-plates of the ambulacra." 



The authors consider that " the Blastoidea constitute a remarkably 

 compact group which is pretty clearly marked oif from the other 

 Pelmatozoa ; " and they point out that the perforate lancet- plate 

 and the regular limitation of the hydrospires to the radial and inter- 

 radial plates, witli their slits parallel to the ambulacra (both points 

 of very considerable importance, as well in a morphological as in a 

 physiological aspect), are characters which are not as yet known to 

 occur in either the Crinoidea or the Cystidea. 



Two orders, six families, and nineteen genera are defined : — 



Order REGULARES, E. & C. 



Pedunculate Blastoids with a symmetrical base, in which the 

 radials and ambulacra are all equal and similar. 



1st Family. Pentremitid^, d'Orbigny (emend. E, &, C). 



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 concen- 

 trated at the lowest part of the radial sinus. 



Fentremites, Say. Pentremitidea, d'Orbigny. 



Mesoblastus, E. & C. 



2nd Family. Troostoblastid^, E. & C. 



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 

 Bide plates. 



Troostocrinus, Shumard. Metahlastus, E. & C. 



TricceJocrinus, Meek & Worthen. 



3rd Family. Nitcleoblastid^, E. &C. 



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-plate and deltoids. 



