MONOGRAPH OF THE EXISTIXG CRINOIDS. 



193 



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

 situated just to the left. The dorsal nerves f the interradial radials and arms are 

 derived from the same sources. 



Thus while the skeletal elements forming the interradial radials and- anus in 

 Thaumatocrinus and Promachocrinus are truly interradial from the very first, all 

 the other elements in their composition are derived by a branching of the elements 

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

 radial radials and arms of these two genera arc primarily twinned reduplications 

 of the equivalent radial series to the left, and must be regarded as having 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 Thaumatocrinus) standing in exactly the same relationship 



FIG. 124. 



FIG. 125 



FIGS. 124-125. 124, LATERAL VIEW OF THE PROXIMAL PORTION OF A SPECIMEN OF ATELECRKVSBALANOIDES FROM BARRA <;RANPE 

 CUBA, SHOWING THE BASALS (AFTER P. H. CARPENTER). 125, LATERAL VIEW OF A SPECIMEN OF ATELECRINUS WYVU.LII 



FROM FIJI, SHOWING THE GREATLY REDUCED BASALS (AFTER P. H. CARPENTER). 



with the paired ambulacra! series as the five axillaries do to tin- ten arms of Antedon, 

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

 position. 



This gives us another reason for regarding the infrabasals as the true starting 

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

 parable to axillaries. The radial pairs of Promachocrinus and of Thaumatocrinus 

 (the primary 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 

 axillary for the reason that the radials are closely crowded into a closed ring, and the 

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

 any point is here rendered impossible. 



Thus Promachocrinus and TTtaumatoeriivus may be described as comatulids 

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



