THE BLASTOIDEA 91 



adjacent ambulacra towards the opposite side. The linear ambulacra, the 



large A, and the strictly pentagonal base remove it from the other 



irregular Blastoids, and suggest affinities with the group of genera last 



discussed. Zygocrinus, Bronn (1848, syn. Astrocrinus, T. & T. Austin, 



1843, non Conrad, 1840, nee Asterocrinus, Miinster), Carboniferous, Britain 



(Fig. XIV.). This highly asymmetrical form 



has been elaborately described by K Etheridge, 



fil. (1876), and by Etheridge & Carpenter 



(1886), but its affinities remain uncertain. 



Its resemblance to Eleutherocrinus lies only in 



secondary characters induced by a sessile life. 



The theca is depressed, sternless, and produced 



into four lobes ; on the shortest of these lies 



an ambulacrum modified much as in Eleuthero- 



crinus, and towards it the two larger BB 



stretch up as in that genus. The four normal 



ambulacra lie in the depressions between the 



lobes. Three A are large and stretch out over 



the lobes; two are small and flank the Etife^T 



abnormal ambulacrum. Anus and spiracles iai ?- Oral view modified 



, ~ , , . ,, .. , food-groove (Arab) towards the 



unknown. One hydrospire-told lies on each observer ; o, peristome ; AS ?, 



side of the radial sinus, and is enclosed by it fette^alfulua? 6 118 ' ther 

 at the distal end (cf. Pentremites, Tricoelocrinus). 



The surface is covered with strong tubercles which bear minute spines 

 (cf. Hystricrinus = Arthracantha, p. 158). 



Classification. We have now reviewed every known genus of 

 Eublastoidea, in an order approximating, as near as our very imperfect 

 knowledge of many types will allow, to that of their race-history. This 

 seems to show three main branches : one leading from Cadaster, through 

 Phaenoschisma, to Pentremitidea and Pentremites, with offshoots Crypto- 

 schisma and Orophocrinus ; the second from Troostocrinus, through Meta- 

 blastus, to Tricoelocrinus, with the offshoot Eleutherocrinus ; the third from 

 Nucleocrinus, through Schizoblastus, to Cryptoblastus, Orbitremites, Meso- 

 blastus, and Heteroblastus, with an offshoot Acentrotremites, and probably 

 Pentephyllum. Zygocrinus also is perhaps connected with this third line 

 of descent. 



The classification of Etheridge & Carpenter does not agree very 

 well with the phylogeny here outlined. The erection of an "Order 

 Irregulares" is no more likely to be correct for Blastoidea than for 

 Crinoidea. With the exception of the Codasteridae, their families of the 

 " Regulares " are based almost entirely on the relations of the hydrospire 

 canals to the deltoids, relations which may vary considerably even in an 

 individual, while they take no account of important differences in the 

 relations of the hydrospires to the ambulacra. Moreover, in the con- 

 struction of family names, these authors have contravened the laws of 

 nomenclature. 



The following classification attempts to overcome the above objections 

 while making as little change as possible : 



Series A. CODONOBLASTIDA. FAMILY 1. CODASTERIDAE. Hydrospire- 



