so.1732. INTERRELATIONSHIPS OF RECENT CRINOIDS— CLARK. 1 17 



junction with them a dorsal cup, here have conic to lie horizontally, 



have moved inward, and have become more or less atrophied or meta- 

 morphosed, taking no pari whatever in the formation of anything bul 

 the bottom of the cup. The Comatulida are sharply divided into t wo 

 parts: (1) Those species in which the stem is retained throughout life, 

 and (2) those in which it is cast off at an early stage — the Pentacrini- 

 tida> and the Comatulida. 



So far as can be judged from the available facts, the earlier stage of 

 these two groups is the same; a dorso-central like that of Phrynocrinus, 

 or a root like that of Rhizocrinus, supports a Bathycrinus-like stem, 

 which in turn bears a crown in which the basals are large and form 

 part of the dorsal cup exactly as in Rhizocrinus, and large orals are 

 present, as in the Holopida and Ptilocrinida generally. But after the 

 formation of a certain number of columnars, approximately definite 

 for each species, a specialized columnar is formed which is united to 

 the preceding by a plane articular surface instead of by an articular 

 surface comprising the usual fulcra! ridge and ligament fossa 1 , and 

 which gives off radially five articulated processes, the cirri. At this 

 stage the basals, at first large and an important and essential pari of 

 the wall of the calyx cup, have considerably diminished in their 

 external size, due to having become more recumbent in position, and 

 to having slipped inward under the central part of the dorsal cup. 



From this point onward the paths of the Pentacrinitidse and the 

 Comatulida diverge. In the former a new series of columnars, resem- 

 bling the first series, is added on top of the cirriferous nodal, this series 

 also terminating, as did the first, in a nodal, a second cirriferous 

 columnar, separated from the first by about the same number of 

 ordinary columnars as separated the first from the dorso-central. 

 This process is continued; new columnars are added one by one, every 

 so often one being formed with a whorl of cirri; with increasing size 

 the columnars gradually become shorter, and the primitive fulcra! 

 ridge little by little spreads out into two fan-like figures, which have 

 indicated radiating lines. The structure 1 of the basals has not changed 

 since the formation of the first nodal and its whorl of cirri: they an 1 

 much reduced in size and lie horizon! ally so t hat their sides are dorsal 

 and ventral, and their edges outward. As each columnar is formed, 

 either directly under the basals or, in later life, by intercalation 

 between the upper columnars, it is cast in a mold formed by the 

 underside of the basals, and thereby becomes modified by the acquisi- 

 tion of petaloid markings, more or less obscuring the primitive radiat- 

 ing lines which otherwise would have been formed. 



In the Comatulida, stem development ceases w hen the first nodal is 

 formed; but the nodal itself greatly increases in size, and usually 

 adds several additional rows of cirri to the first; coincident all v the 

 basals slip farther and farther inward, disappearing from sight alto- 



