MONOGRAPH OF THE EXISTING CRINOIDS. 



Ill 



first element of this pair is always missing, so that the latter are 3(2+3), or 

 occasionally 1. 



The suppression of the first element in the first brachial pair and its redupli- 

 cations occurs from the pair immediately following the IIBr axillary onward, 

 regardless of whether the pairs be incorporated in a division series or form the 

 basis of an arm. The division series and arms following IIBr series of 2 do not 

 differ from those following IIBr series of 4(3+4). 



In CapUlaster and Nemaster we have normally IBr series of 2; IIBr series 

 of 4(3+4), rarely of 2; IIIBr series (in which the first ossicle is suppressed) of 

 3(2+3), or 1. In addition to these we sometimes find division series of 2(1+2) 

 where in the IIBr series the two ossicles have been exceptionally closely joined, 

 and where in the IIIBr and fol- 

 lowing series of the normal type 

 4(3+4) both elements of the first 

 pair have been suppressed, leav- 

 ing a division series composed of 

 the syzygial pair only, 2(1+2). 

 In the arms as well as in the outer 

 division series both elements of 

 the first brachial pair may be 

 omitted, so that the arms may be- 

 gin with a syzygial pair. 



Bearing in mind the relative 

 instability of division series of 

 more than two elements as a re- 

 sult of the constant tendency to 

 a reduction in the number of 

 their elements to the more primi- 

 tive two, and the fact that no 

 fewer than five types of division 

 series, 4(3+4), 3(2+3), 2(1+2), 

 2 and 1 are of normal occurrence in these genera, we are prepared to find individ- 

 uals of the component species in which extraordinary variability occurs. 



P. H. Carpenter records a specimen of a species of Nemaster closely allied 

 to N. ribiginosa which was dredged by the Blake in the Caribbean Sea in which 

 the IIIBr series are 1, and the IVBr series may be 1, or 2(1+2) ; the second 

 and third brachials are usually united by syzygy on the outer arms of each ray, 

 and sometimes also on the adradial arm which is on the inner side of each IIBr 

 series ; but the other arms generally have the first two brachials united by syzygy. 



The explanation of this condition is very simple. Whereas usually the 

 IIIBr and IVBr series in this genus are 4(3+4) minus the first ossicle, leaving 

 3(2+3), in this case they are 2 minus the first ossicle, leaving 1; the IVBr series 

 of 2(1+2) are series of 4(3+4) minus the first two ossicles; and this feature is 

 characteristic of most of the arms where both of the elements of the first brachial 



190 



FIG. 190. LATERAL VIEW OF TYPE SPECIMEN OF EUDIO- 

 CRINUS OHXATUS. 





