CLASS II 



BLASTOIDEA 



16^ 



' The last-named authors subdivided the Blastoids into Begulares and 

 Irregulares, an arrangement representing incidental variation rather than any- 

 broad morphological differentiation. That presented by Bather in Part III. 

 of Lankester's Zoology, 1899, appears to be more logical, and is followed in 

 principle here. By separating the typical Blastoids, in which the character- 

 istic calyx plates have become fixed at a small and definite number, from the 

 earlier forms which have not attained that structure, two main divisions may 

 be recognised, viz. : Protohlastoidea and EuUastoidea. Hudson, whose admirable 

 studies upon Blastoidocrinus have thoroughly elucidated that hitherto obscure 

 type, has suggested a third, ParaUastoidea, to express the differences shown by 

 his researches between it. and the other Ordovician forms. As the general 

 division is a somewhat arbitrary one at best, it is thought that these differences 

 are sufficiently emphasised by the family diagnosis. 



Order 1. PROTOBLASTOIDBA Bather (emend.) 



Calyx plates numerous, not limited to a definite number. 



Family 1. Asteroblastidae. 



Blastoidea with calyx plates indefinitely arranged above basals and radials, and 

 having, along with pentamerous ambulacra 

 and marginal brachiole^s, diplopores and 

 pore-plate, but no hydrospires. Ordovician. 



JsteroblastusEichw. (Fig. 259). Calyx 

 gemmiform, pentagonal, pedunculate, and 

 composed of numerous rigidly united 

 plates which are perforated by conjugate 

 pores. Upper surface marked by five 

 large petaloid or stellate areas which 

 are occupied by alternating plates, and 

 bordered by sockets for the attachment of brachioles. Ordovician ; Russia. 



Fig. 250. 



Asterohlastus stellatus Eichwald. Ordovician ; 

 Pulkowa, Russia. Natural size (after Schmidt). 



Family 2. Blastoidocrinidae Bather (emend.). 



Calyx plates more definitely arranged in four circlets, without diplopores or pore- 

 plate, but with hydrospires present. Ordovician. 



Blastoidocrinus Billings (emend. Hudson). Calyx pentagonal, composed of 

 four circlets of principal plates, viz. : (1) basals (number unknown); (2) 

 radials with angular distal face, followed by (3) two large plates called bi- 

 brachials, with numerous interbrachials in each interradius ; and (4) very 

 large triangular deltoids, with hydrospire-slits at their lower margins 

 Adambulacrals, heavy covering plates, and some additional plates in the oral 

 portion. Ambulacra large, bordered with numerous brachioles or pinnides. 

 Base invaginate, with strong column occupying the concavity. Ordovician 

 (Chazy Group) ; Canada and New York. 



Order 2. EUBLASTOIDEA Bather. 



Calyx plates limited to a definite number of about thirteen. Hydrospires always 

 present. 



