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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXV. 



Fig. 26. — Pentacrinitid2t: 

 Endoxocrinus. 



tion, Carpenter" mentions a specimen of Phanogeniaalata ("Actino- 

 metra pulchella' 1 '') in which one of the radials is an axillary," sup- 

 porting two post-radial series, and I have recently recorded a speci- 

 men of Heliometra maxima which presents the same condition ; more- 

 over it is probable that Promachocrinus and 

 Decametrocrinus originally came into ex- 

 istence through a division of the muscular 

 articulation on the distal end of the radial, 

 which later became more and more firmly 

 fixed, finally resulting in a division of the 

 radials themselves, so that the two genera now 

 have ten radials instead of the original five. 

 If this were true Ave should expect reversions 

 to occur, and Promachocrinus to sometimes be 

 found with one or more radials single instead 

 of double, and bearing a post-radial series 

 comparable to those in Heliometra, the most closely allied genus ; and 

 Decametrocrinus to occasionally occur with fewer than ten rays, thus 

 approximating the most nearly related genus, Pentametrocrinus ; 

 and it is somewhat remarkable that, considering the small number 

 of specimens representing species 

 of these two genera which has 

 been discovered, one, the type of 

 Decametrocrinus rugosus, should 

 be only nine armed, through the 

 persistence of one entire radial 

 (the right posterior), and the 

 division of the remaining four. 



Isocrinus (fig. 27). — In Iso- 

 crinus naresianus we find a con- 

 dition exactly similar to that de- 

 scribed for the Atelecrinida? and 

 Antedonida? ; Z x and Z, are the 

 third and fourth post-radial 

 joints, or the first two joints fol- 

 lowing the axillary. In Isocrinus 

 ivyville-thomsoni, I. parrce ( = 

 Pentacrinus mulleri -\- P. mac- 

 learanus), I. alternicirrus, and /. 

 sib ogee (fig. 26) Z x and Z 2 are the 

 first and second joints of the free undivided arm. The arm structure 

 is therefore similar to that described for the comatulids with more 

 than ten arms, excepting those in the genus Comastcr; in these species 



Fig. 27. — I'entaciiixitid.e ; Isocrinus. 



a Challenger Reports, XXVI, Zoology, p. 27. 



