CLARK: THE CRINOID GENUS HOLOPUS • 137 



irocrinus — even to the detail of the slight eversion of the edges 

 of the orals. 



So far as I can see, the column of Holopus is composed of 

 radials only. In the young specimen figured b}^ Alexander 

 Agassiz the uniformity of the ornamentation on the outer ring 

 appears to indicate that it is composed of a single series of plates, 

 which must be the radials. In one of the specimens figured by 

 P. H. Carpenter- the series of tubercles running down the median 

 line of each sector of the column indicates that the same plate 

 (the radial) persists as far as this ornamentation extends, and 

 probably also to the circumference of the basal disc. 



An analysis of all the available characters^ indicates that 

 Holopus occupies practically the same developmental plane as 

 the pentacrinites and the comatulids; indeed it is questionable 

 which of the three groups should be considered the most special- 

 ized. 



My personal opinion is that the pentacrinites, the comatulids, 

 and Holopus are very closely related, in spite of their extraordinary 

 superficial dissimilarity. 



In the pentacrinites the column is enormously developed; so 

 rapid is the growth that the proximales as they are continuously 

 formed beneath the calyx never succeed in becoming attached 

 to it, but are continuously pushed outward by the formation 

 of new proximales between the last formed and the calyx; the 

 proximales later become separated by the intercalation of other 

 columnals, appearing in the fully developed column as the cirrif- 

 erous nodals. The basals are much reduced and lie horizon- 

 tally. 



In the comatulids a short column is formed and a proximale 

 appears which, becoming firmly attached to the calyx, increases 

 enormously in size and, the larval column being discarded, con- 

 tains the entire adult stem. The basals, in nearly all the types, 

 become metamorphosed into an internal septum and entirely lose 

 their original character. The base therefore is entirely com- 



- "Challenger" Report, Stalked Crinoids, 1884, plate III, fig. i. 



' Phylogenetic study of the recent crinoids , Smiths. Misc. Coll. 65: No. 10. 



August 19, 1915- 



