THE BLASTOIDEA 



81 



preserved, as in Asteroblastus and many Eublastoids. These known facts 

 afford no character, other than the less development of the hydrospires, 

 by which Blastoidocrinus should be separated from the Eublastoidea, so 



br 



sp 



br- 



Fio. ill. 



Blastoidocrinus carchariaeilens ; restored on the evidence of E. Billings. 1, oral surface ; 3 

 -ays show the side-plates (sp) with their facets for brachioles (br 1 ) ; 1 ray shows the brachioles 

 (br) attached ; the 5th ray shows lancet-plate (L) exposed by removal of side-plates. 2, from 

 the side ; the upward extension of the stem (St) and the invagiuated basals (B) are indicated by 

 dotted lines, h, incipient hydrospires. 



long as that order includes such a form as Cadaster. But till the structure 

 of the base and the position of the anus are known, the genus may be 

 kept with its rather more primitive allies, Asteroblastus and Asterocystis. 



GRADE B. Eublastoidea, Bather (1899). 

 ( = BLASTOIDEA, A-uctt.). 



Blastoidea in which the thecal plates have assumed a definite 

 number and position in three circlets, as follows : 3 BB, 2 large 

 (formed by fusion of two pairs of the primitive 5 BB) and 1 small, 

 in r. ant. IR ; 5 RR, often fork-shaped, forming a closed circlet ; 

 5 A, interradial in position, supported on the shoulders or the pro- 

 cesses of the RR, and often surrounding the peristome with their 

 oral ends. The stereom of the RR and A on either side of the 

 ambulacra is thrown into folds running across the radio-deltoid 

 suture ; these folds hang down into the thecal cavity, forming the 

 hydrospires. 



Beginning in the Silurian with Cadaster and Troostocrinus, we 

 may trace the gradual modification of a simple type,, and the 

 evolution of the numerous complicated structures characteristic of 

 so specialised a form as Pentremites. Starting afresh with Elae- 

 acrinus we shall study the elaboration of the structures that 

 characterise Orbitremites ( = Granatocrinus), which represents the 

 acme of the Eublastoidea in Britain. Thus the morphology and 

 the classification will be unfolded along with the phylogeny. 



6 



