1 36 INVERTEBRATE PALAEONTOLOGY 



typing of symmetry on the pentamerous plan, and local 

 specialization of the plates traversed by the straight 

 food-grooves. Both of these genera are almost as 

 much Blastoids as Cystids; Protocrinus bears no small 

 resemblance to some Edrioasteroids. 



The last-named class (regarded by many as an order 

 of Cystidea) seems to have been either extremely sessile 

 (without stems), or imperfectly eleutherozoic (after the 

 manner of limpets). Edrioasteroidea range from the 

 Cambrian to the Carboniferous, and have been claimed 

 as transitional between Pelmatozoa and Eleutherozoa. 

 That they have strong Cystid (even Amphoridean) 

 affinities seems certain, but in ambulacral structure 

 they are definitely unlike the Pelmatozoa, and show a 

 marked resemblance to eleutherozoic types. Stromato- 

 cystis is a Cambrian genus ; Edrioaster typically Ordo- 

 vician. Hemicystis^ in the Silurian, foreshadows the 

 scaly types characteristic of the Upper Palaeozoic. 



The class of small Pelmatozoa named Blastoidea 

 comprises forms that developed along " crinoidal ". lines 

 except in ambulacral characters. Instead of being 

 raised on flexible arms, the food-grooves are recumbent, 

 and pass with extreme regularity down five radii from 

 the oral pole, sometimes almost reaching the opposite 

 extremity (whence projects a slender stem). These 

 "ambulacra" show strange superficial resemblance to 

 the areas similarly named in Echinoidea; the com- 

 parison is amplified by the development of simple 

 and "petaloid" structures. Primitive Blastoids (Proto- 

 blastoida) are restricted to the Ordovician, and possess 

 some diploporous plates, thus demonstrating their affinity 

 with Cystid Diploporita. True Blastoids appeared first 

 in the Silurian, where Cadaster showed an early and 

 incomplete phase of the development of internal stereom- 

 folds (hydrospires) characteristic of typical families. 



