KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 



25. n:o 2. 



61 



Catillocrinus and, on iny interpretation, Pisocrinus (text-tig. 2), Triacrinus (text-tig. 3) 

 and Calycanthocrinus (text-tig. 7): in all of these, as well as in the above-mentioned 

 forms, the two large arm-bearing radials are the left posterior and the anterior, and this 

 agrees with the proposed reading of the Calceoerinida>. In short, by the ehanges here 

 made, the whole of the monocyclic Inadunata, whenever they differ from the symmetrical 

 pentamerous type, differ according to one general plan. Messers Wachsmuth and Springer 1 ) 

 have stated that among the Heterocrinida3», one of the compound radials is »exceptionally 

 the anterior one in place of the left anterolateral», but sueh cases are too rare to affect 

 the argument. The import ance of the above conelusion will be seen by all students of the 

 Crinoidea, but this is not the place to dilate on its eonsequences (see antea, p. 20). 



Turning again to the diagram of Castocrinus, it is seen that the absent basal is the 

 right posterior; whether it has disappeared by atrophy, has fused with the right anterior 

 (as in Triacrinus &c.) or has merely escaped observation, one cannot say. Applying to 

 the right posterior radius the terminology used for other Inadunate genera, Ave shall call 



Fig. 13. Diagrams illustrating the structure of the posterior area in 



a) Castocrinus. 



b) Euchirocrinus. 



c) Calceocrinus. 



(i) Halysiocrinus. 



the plate supported by the super-radial the Anal %', while the infer-radial will be 'the 

 Radianal'. 



We are now in a position to extend our terminology to the more specialised genera 

 of the family, first of which comes Euchirocrinus. Here the two halves of the left an- 

 terior radial still exist and are still connected; but the infer-radial is narrowed, especially 

 in its distal region, while the super-radial has its upper margin fiush with the upper 

 margins of the large radials. The anterior and left posterior radials are increased in width. 

 The right posterior super-radial (Radianal) and the right anterior super-radial still exist, 

 but are a little smaller. The arm of the right anterior radius, which was the smallest 

 in Castocrinus, has now entirely disappeared. The ventral tube, on the contrary, which 

 in Castocrinus was supported only by the right posterior radial, has now shifted to a 

 position more symmetrical with the position of the calyx, so that the anal x rests on the 

 right posterior and left anterior infer-radials. These infer-radials, which in Castocrinus 

 were already very closely united, have now entirely fused and have at the same time 

 become smaller; they thus form a single plate of broad T-shape, which still bears to the 



l ) »Perisomic Plates &c», loc. cit., p. 378. 



