ECHINODERMA. BLASTOIDEA 155 



the forked parts sharply bent, forming part of the flattened upper 

 surface of the calyx ; deltoids and ambulacra confined to upper 

 surface. A long lancet-plate, with side-plates, occurs between the 

 deltoids and radials. Hydrospires consist of sharp folds of the 

 calyx where the radials and deltoids meet, and open at the surface 

 by slits. Mouth pentagonal, originally plated over ; no spiracles ; 

 anus between the posterior deltoid and radials. Silurian to Carboni- 

 ferous. Ex. C. trilobatus, Carboniferous. 



Orbitremites { = Granatocrinus). Calyx elliptical, ovate, or 

 more or less spherical, in section pentagonal or round ; with concave 

 base. Basals small, not seen in a side view. Radials of variable size 

 and forming part of the base. Deltoids generally rhombic, large in 

 some species, small in others. Ambulacra narrow, straight, with 

 nearly parallel sides. Lancet-plate narrow. Hydrospires simple, 

 usually with two or three folds only, dilated at the free ends ; the 

 inner fold forms a plate next to the lancet-plate. Spiracles five, 

 round or oval, piercing the apices of the deltoids, the posterior one 

 including the anus. Carboniferous Limestone. Ex. 0. derbiensis. 



Distribution of the Blastoidea 



In England the Blastoids range from the Devonian to 

 the Carboniferous, being most abundant in the latter. 

 A few primitive types (Asteroblastus, Blastoidocrinus) 

 occur in the Ordovician of Russia and Canada ; and some 

 others (Troostocrinus, Codaster) are found in the Silurian of 

 North America. The English Devonian forms are rare 

 and but little known. In the Carboniferous Limestone 

 the blastoids attain their maximum development ; ten 

 genera are represented, the most important being Codaster, 

 Orophocrinus, Schizoblastus, Orbitremites and Mesoblastus. 

 Pentremites is common in the Carboniferous of America, 

 but is not found in Britain. 



