20 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



plates. Such is the case in Pleurocystis, where the definite skeletal 

 elements are restricted mainly to the upper surface and extend below 

 as a rule only as marginals. 



In Cheirocrinus this process has not been carried so far, but in 

 such a form as Cheirocrinus penniger (PL 2, figs. 1, 2) it is to be 

 noted that there is an appreciable flattening of the theca and a 

 remarkable enlargement of the circum-anal area. Scoliocystis seems 

 to show an intermediate stage in the evolution of this type of struc- 

 ture, comparable in degree with that shown by Cheirocrinus wal- 

 cotti. 



Pleurocystis. — Pleurocystis undoubtedly led an eleutherozoic exist- 

 ence, and one, as elsewhere suggested, quite at variance in certain 

 respects Avith that to be found in the case of most of the detached 

 Pelmatozoa. All the structural evidence afforded by the genus 

 points clearly to the assumption by the form of a purely prostrate 

 habit. For this type of life Pleurocystis is only excelled in the per- 

 fection of its adaptation by the members of the next subgroup, 

 which doubtless represent the constant assumption of this sort of 

 existence for a long period of time. 



An examination of specimens of Pleurocystis, or even of figures, 

 such as figure 4, Plate 2, and figure 3, Plate 3, will clearly show the 

 extent of these modifications. The marked asymmetry of the genus 

 which can only be accounted for on the basis of the assumption of a 

 prostrate habit, is a most peculiar and striking feature. One notes 

 the extreme flattening of the anal (ventral) side and the strong con- 

 vexity of the antanal side; the atrophy of all but two of the food 

 grooves; the location of all the pectini-rhombs on the dorsal convex 

 side; the extreme posterior location of the anal opening; the fact that 

 the column is constantly coiled in the same plane as that of the flat- 

 tened theca; and other characteristic features. All these facts point 

 to the same conclusion. 



A free or semifree type of existence could be of little benefit to an 

 animal were it not for the possession of organs of locomotion. Possi- 

 bly the brachioles of Pleurocystis might have been used in somewhat 

 the same manner as the flagellse of certain Protozoa. By a lasliing 

 or rowing motion of these organs it is quite conceivable that locomo- 

 tion might have been effected. The brachioles are too long, slender, 

 and flexible to fvmction as ambulatory appendages, but they may 

 have caught the bottom laterally and in this way have served as 

 organs of propulsion. The column in this group probably in no case 

 helped give impetus to the animal's movements as seems to have 

 been the case in the next subgroup. 



The column of Pleurocystis is comparatively short and quite stout 

 in the proximal portion. Distally it tapers rapidly until it becomes 

 remarkably slender, Plate 3, figure 3. The distal two inches or so of 



