THE BLASTOIDEA 



folds distinctly portions of the thecal plates, coming to the surface of the 

 radial sinus. No distinct hydrospire-canal or pores ; spiracles developed 

 imperfectly or not at all. Genera Cadaster, Pluienoscldsma, Cryptoschisma, 

 Orophocrinus. This family coincides with that of Etheridge & Carpenter. 

 FAMILY 2. PENTREMITIDAE. Hydrospire-folds, usually numerous, concen- 

 trated at the lowest part of the radial sinus, and partly or wholly pendent. 

 Hydrospire-canal opens through spiracles bounded distally by side-plates. 

 Base convex. Ambulacra rather broad. Genera Pentremitidea, Pentre- 

 mites. This equals the Pentremitidae of Etheridge & Carpenter, minus 

 Mesoblastus. 



Fio. XV. 



Thecas of typical Blastoids. 1 , a Codonoblastid Pentremites robustus (from Brit. Mus. E8145). 

 2, a Troostoblastid Tricoelocrinus woodmani (from Brit. Mus. E8171). 3 and 4, a Granato- 

 blastid Orbitnmites orbicularis (from the type-specimens, Brit. Mus. ES135). 3, from the side. 

 4, from below, with posterior interradius uppermost. All figures nat. size. 



Series B. TROOSTOBLASTID A. FAMILY 1. TROOSTOCRINIDAE. Elongate 

 forms with linear ambulacra descending sharply outwards from the much 

 restricted peristome. Hydrospire-folds only slightly concentrated, but 

 communicate with exterior through pores, and through spiracles bounded 

 by A and lancet-plates. Genera Troostocrinus, Metablastus, Tricoelocrinus. 

 This equals the Troostoblasiidae of Etheridge & Carpenter. FAMILY 2. 

 ELEUTHEROCRINIDAE. Elongate, stemless, asymmetrical, with four narrow 

 ambulacra, accompanied by unconcentrated hydrospires. Fifth ambula- 

 crum shortened and widened. A minute. Genus Eleutherocrinus. 



Series C. GRANATOBLASTIDA. FAMILY 1. NUCLEOCRINIDAE. Interam- 

 bulacra show traces of a primitive triad of plates. Ambulacra linear, and 

 stretching far down the theca, which is ovoid. Hydrospire-folds few and 

 pendent. Spiracles double. Mouth roofed by large plates firmly united 

 into a tegmen. Genera Nucleocrinus, Schizoblastus. This family equals 

 Etheridge & Carpenter's Nucleoblastidae, minus CryptoUastus and Acentro- 

 tremites. FAMILY 2. ORBITREMITIDAE. Theca globular with concave or 

 flattened base. Ambulacra linear, stretching down to concavity of base. 

 Hydrospire-folds few and pendent ; a hydrospi re-plate always present 

 (unknown in Heteroblastus). Hydrospire-folds rarely penetrate A, but long 

 canals pass onward, through, beside, or under them, except in Acentro- 

 tremites. Genera Orbitremites, Cryptoblastus, Heteroblastus, Mesoblastus, 

 Acentrotremites. This corresponds to Etheridge & Carpenter's Granato- 

 blastidae, plus Cryptoblastus, Mesoblastus, and Acentrotremites. FAMILY 3. 



