56 UmTACRINUS: ITS STEUCTUEE AKD EELATIO^^S. 



Uintacrinus certainly represents a parallel development. In addition to 

 the free-floating habit, it has the disk of one of the Comatulid genera, such 

 as no other Crinoid is known to have, and also its open ambulacra, arms, 

 and pinnules. Furthermore, AcUnometra is now generally considered to be 

 a dicyclic — or rather pseudomonocyclic, — or " cryptodicyclic " — Crinoid, 

 although infrabasals have never been seen in it. Mr. Bather classifies it 

 among the Dicyclica in the Lankester Zoology. It is therefore a legiti- 

 mate assumption that at some stage of its larval development it possessed 

 infrabasals, and also a stem (which indeed we know it had), with the top 

 columnal of which they fused, and which disappeared, leaving this top colum- 

 nal as a relic — the so-called centrodorsal. In what order these conditions 

 occurred, of course we do not know ; but as a consequence of them it fol- 

 lows that at some period, or successive periods, of its growth, the base of 

 Actinometra contained the same — or equivalent — elements as Uintacrinus. 

 Obliterate the interbrachial system of Uintacrinus, and we would have some 

 stage of Actinometra ; conversely, by adding the interbrachials, and sup- 

 posing some larval stage or stages of the base of Actinometra to become 

 persistent, we would produce Uintacrinus. 



And this suggests that it would be a most important thing if somebody 

 could work out the life history of Actinometra, as W. B. Carpenter, Wy ville 

 Thomson, and Bury have done for Antedon. There are several species 

 which seem to be sufficiently littoral in habitat to make this practicable. 

 The ^^ pentacrinoid " stage of Act. meridionalis has been seen. It was brought 

 up by the '' Blake," and described by P. H. Carpenter ^ substantially as 

 follows : — 



Actinometra meridionalis : Two pentacrinoids were found among the '^ Blake" 

 collections, one at about the stage represented at Fig. 1 B, PI. XXXIX., of 

 W. B. Carpenter's work on Antedon rosacea, and the other by Fig. 1 C of the 

 same plate. The centrodorsal is scarcely larger than the stem joints 

 immediately below it ; five of these are short and discoidal, and the next 

 miuch elongated.! 



The displacement of the mouth in Actinometra from the central position 

 which it occupies in the tegmen of the Crinoids generally, represents a 



* Bull. Mus. Comp. Zool, IX., No. 4, p. 14. 

 f See also Agassiz's Seaside Studies, p. 121. 



