MONOGRAPH OF THE EXISTING CRINOIDS. 



193 



the large radial branches already existing leading to the radial post-radial- series 

 situated just to the left. The dorsal nerv(>s of the interradial radials and arms are 

 derived from the same sources. 



Thus while the skeletal elements fornimg the interradial radials and- arms in 

 Thaunwtocrinus and Promachocrinus arc truly interradial from the very first, all 

 the other elements in theu- composition are derived by a branching of the elements 

 leading to the radial radials and arms to their left. It follows, therefore, that the mter- 

 radial^adials and arms of these two genera are primarily twhmed reduplications 

 of the equivalent radial series to the left, and must be regarded as havmg exactly 

 the same relationship with the radial series to their left as the two arms of each arm 

 pair in Antedon have with each other, each of the five infrabasals of Promachocrinus 

 (and presumably also of Thaumatocnnus) standing in exactly the same relationship 



Fia. 124. 



Fia. 125 



FIGS 124-125 -124 LATERAL VIEW OF THE PEOXIMAI. PORTION OF A SPECIMEN OP ATELECRINUS BALAN0IDE8 PBOM BABBA GRAOTE 

 ciBA SHO-i^G TI.E BASAI.S (APTER P. H. CARPESTER). 125, LATERAL VIEW OP A SPECIMEN OP ATELECR»f S ^T.U1 



PROM Fiji, showing the greatly reduced basals (after P. H. Carpenter). 



with the paired ambulacral series as the five axiUarics do to the ten arms of Antedon, 

 though not, on account of mechanical considerations, quite comparable m relative 



^°^^ This gives us another reason for regardmg the infrabasals as the true starting 

 point of the radial series in the crinoids, and for regarding the radials as qmte com- 

 parable to axiUaries. The radial pah's of Promachocrinus and of Thaumatocrmus 

 (the primarv radials and the interradial radials to their right) should probably 

 each be regarded as the equivalent of an axillary which is unable to appear as an 

 axillarv for the reason that the radials are closely crowded into a closed rmg. and the 

 separation of the following series necessitated by the formation of an axillary at 

 anv point is here rendered impossible. , , ■, , , ,:j„ 



■ Thus Promachocrinus and Thaumatocrinus may be described as comatuhds 

 with five doubled radial series, in which the skeleton of the five later series arises 



