MORPHOLOGY . 49 



perisome, which is traversed by the five radial grooves (ambulacra) passing 

 out from the peristome between the oral valves to the bifurcation of the arms. 

 The same condition is shown in one stage of the larval Comactinia hereinafter 

 described. This condition is also comparable to that of Holopus (PI. A, fig. 8), 

 and of Thaumatocrinus (PI. A, fig. gb), as the orals would appear with some 

 further growth of perisome around them. 



A more striking parallelism is found in the tegmens of some genera of the 

 family Plicatocrinidae, which has been shown 1 to possess characters approxi- 

 mating those of the Paleozoic Inadunata more closely than any other of the 

 living crinoids. In these forms the orals persist in the adult stage as a central 

 pyramid surrounded by perisomic plates of varying size, some quite large and 

 distinct, thus completely enclosing the visceral mass except for the open mouth 

 and ambulacra. Among these is Hyocrinns, which as figured by P. H. Car- 

 penter in the Challenger Report on the Stalked Crinoids, plate 6, has rather 

 large triangular orals, and a lateral anal tube fairly low down at the margin of 

 the disk. Thalassocrinus (A. H. Clark, Monograph of the Existing Crinoids, 

 p. 209, fig. 145) has a similar pyramid, with the apex decidedly high, the edges 

 of the orals curiously projecting in thin ribbon-like, sinuous margins which are 

 repeated in the covering plates of the ambulacra throughout the arms ; it also 

 has quite large interradials, few in number. 



Still more interesting for comparison with our Flexible tegmen is that of 

 the type species of Ptilocrinus from the North Pacific, of which I give an 

 original figure (PI. LXXV, fig. ic). Here not only are the orals with their 

 large connecting ambulacra deeply surrounded by perisomic plates of good 

 size, so that they occupy a relatively small space at the center, but they are 

 somewhat asymmetrical owing to the influence of strong plates surrounding 

 the anal opening, which is rather high up in the interambulacral area. 



The orals of the Flexibilia are unequal, the posterior plate being dispro- 

 portionally enlarged, sometimes almost exceeding in area the other four com- 

 bined. It is a madreporite perforated by numerous pores, as is shown in 

 Onychocrinus (PI. LXVII, fig. 9^), and especially by the perfectly preserved 

 plate in Syuerocrinus incurvus (PI. XLII, figs. 8n, 0) ; and it doubtless per- 

 formed the functions of the perforated orals seen in Holopus (PI. A, fig. 8), 

 and in Hyocrinns as described by Carpenter (Challenger, Stalked Crinoids, 

 p. 95). The enormous development of the posterior oral does not find a 

 parallel in any of the Recent tegmens, which so far as known are in this re- 

 spect perfectly symmetrical, save for the slight irregularity in Ptilocrinus. 

 But this kind of irregularity is the rule in the orals of the Camerata and the 

 Inadunata. 



1 Clark, A. H., Jour. Washington Acarl. Sci., 1913, p. 494. 



