310 



PHYLUM ECHINODERMATA. 



A s 



and probably with pinnules attached to the covering plates ; the 

 pinnules distinguish them from Echinoids which they otherwise resemble 

 (Fig. 221). The ambulacra! grooves extend almost to the aboral pole, and 

 there are furrows passing between the covering plates which may have 

 served for tube -feet ; they do not appear to have had a stalk but they 



may have been fixed by the aboral 

 pole. Asteroblastus Eichw. (Fig. 215), 

 U. Cambrian. Blastoidocrinus Billings, 

 U. Cambrian. The two last named 

 genera are placed by Bather with the 

 Blastoids. 



Order 4. EDB.IOASTERTDA. 

 With radial symmetry and, in some 

 forms, pores between the ambulacra! 

 plates as though for the passage of tube- 

 feet ; the theca is composed of irregular 

 plates, the madreporite is well marked 

 and lies near the mouth in the same 

 interradius as the anus (Fig. 222) ; no 

 pinnules. There does not appear to 

 have been a stalk, though in some 



cases they may have adhered by the aboral surface. They present some 

 resemblances to Asteroids. U. Cambrian to Carboniferous. Cystaster Hall 

 (Fig. 211), U. Cambrian. Edrioaster Bell., U. Cambrian (Fig. 222). 

 Agelacrinus Vanuxem, U. Cambrian, Silurian, Devonian, Carboniferous. 



FlO. 221. Mesocystis Pusirefskii (re- 

 stored after Hoffmann and Nitikin, 

 from Bather). As anus ; M water- 

 pores or perforations caused by a 

 parasite ; mouth. 



amb - 



amb 



P amb 



FIG. 222. Edrioaster Bigsbyi (after Bather). 1 oral view. 2 section through a radius and 

 interradius of the same specimen. 3 Section across an ambulacrum. ad flooring 

 plates of the ambulacra! grooves ; amb covering plates of grooves ; As anus ; ia inter- 

 ambulacral plates ; M madreporite ; p pores between the ambulacra! plates ; ps peri- 

 stome with covering plates ; v.g ventral groove. 



Class BLASTOIDEA.* 



Pentamerous forms without arms, with radial ambulacra bearing 

 pinnules, with a well-plated monocyclic calyx, with hydrospires, 

 without pores. 



The Blastoids are entirely extinct, being confined to the Palaeozoic 



* Etheridge and Carpenter, Catalogue of the Blastoidea in the Geological 

 Department of the British Museum, 1886. Bather, op. cit. ; Zittel, op. cit. 



