102 NEW YORK STATE MUSEUM 



an arm. The first and second circlet of similar forms have 

 however received crinoid terminology and I have used similar 

 terms for the plates of the third and fourth circlets. These last 

 circlets lie in the brachial region of crinoids. The bibrachials 

 here follow the radial as if that were a primaxil and the pair 

 suggest the II Br^ of crinoids with their long axes placed verti- 

 cally instead of horizontally. I do not mean to lead the reader 

 to conclude that I have considered these plates as divided radials 

 though a comparison with the higher blastids might hastily 

 lead one to that conclusion. Three other hypotheses suggest 

 themselves which may be here given without comment. These 

 plates may be homologous with the CA^stidean pair of plates that 

 would very naturally lie over what became a radial (through 

 reduction of number and acquisition of pentamerous symmetry), 

 possess a vertical common suture and become modified on being 

 reached by the extending oral food grooves. That is, these 

 plates may be considered strictly interradial and without radial 

 elements between them. In this case they are but specialized 

 plates similar in origin to the other plates of the third circlet. 

 The double character of these plates and their position at the 

 end of a long double row of highly specialized adambulacrals 

 might lead us to adopt the hypothesis that they were derived 

 from the outermost adambulacrals of such a form as Protero- 

 blastus on which a more distinct differentiation into oral and 

 aboral surfaces had been impressed together with loss of func- 

 tion at the aboral end of the arm. A third hypothesis, and one 

 perhaps more suggestive than the last, would be to look on these 

 bibrachials as true interambulacrals thrust from the oral sur- 

 face and to their present radial position by the great develop- 

 ment of the deltoids but still bearing brachioles and outward 

 markings indicating a former respiratory function. 



Interbrachials. In each interradius there are two or three 

 interbrachials of the third circlet, one much larger than the other, 

 and from lo to 12 smaller plates of the fourth circlet. The upper 

 ends of the latter are each in contact with the middle part of the 

 great deltoid above. They no doubt once functioned as respira- 

 tory plates and to a certain extent they may still do so though 

 any such present function is unknown. The arrangement, shape 

 and relative size of these plates is shown in figure i of the text 

 as is also their contact with the bibrachials and radials. Plates 

 2 and 3 show four different groups of these interbrachial plates 



