10 Trans. Acad. Sci. of St. Louis. 



proportions of the actiual and abactinal regions among 

 Ecliinoderms were looked upon by Prof. L. Agassiz as 

 determining the different outlines of the various ' orders of 

 this class,' and he has ranked their orders according to the 

 greater preponderance of the one or the other of the two re- 

 gions. In the Neocrinoidea, the oral and aboral regions are 

 proportioned almost equally, and this is the case not only in 

 the adult, but is to be observed already in the Pentacrinoid- 

 larva. In the Palaeocrinoidea, the abactinal regions, as a 

 rule, are considerably contracted, and in the lower organ- 

 ized Blastoids they are reduced still more. Plates of such 

 enormous dimensions as are found occasionally among the 

 deltoids, cannot possibly form a part of the actinal system 

 in so low a group as the Blastoids, and hence cannot be 

 orals. That they are true interradials is proved by the 

 relative position which they occupy to the interradials of 

 the Palaeocrinoideae. Like those plates, they rest upon the 

 upper sloping sides of the radials, and extend, whether 

 consisting of a single plate as in the Cyathocrinidae, or of 

 a series of pieces as in Actinocrinidae and Rhodocrinidae, 

 into the ventral side, to a series of plates, which were desig- 

 nated by W. and Sp. as central pieces and proximals." 



The first question which presents itself is, what is meant 

 by interradial? The word undoubtedly means something 

 between the rays of a circle. We therefore have to assume a 

 primary circle of rays or plates between which the secondary 

 parts can be placed, as, for example, in Crinoids those plates 

 which are placed between the regular radial plates. The 

 second question would be, what is the function of the inter- 

 radials? Eef erring to Crinoids again we find that their 

 function is to fill out space in order to increase the circumfer- 

 ence of the body. They are not essential to the construction 

 of the body or calyx as is well illustrated in a number of 

 Crinoids which have no interradials. Besides, their number 

 is variable. Therefore they may be regarded as supernu- 

 merary pieces whose only function is to enlarge the body. In 

 no case do they enter into the construction of the various 

 openings unless we regard the anals as interradials, which I 



