14 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



rate. Measurements of the stem of a Pleurocystites flitextus will 

 illustrate these features admirably. The specimen is an unusually 

 large one, and the stem as preserved measures 70 mm. in length. 

 Probably not more than 5 or 10 mm. of the stem is wanting. Pos- 

 sibly the column is essentially complete. 



The proximal columnal measures 6.1 mm. in diameter and the most 

 distal columnal preserved 1.2 mm. The stem tapers most rapidly 

 in the first 16 mm. where it decreases from 6.1 to 4 mm. in diameter. 

 Distad from this point the rate of decrease is lower and compara- 

 tively uniform. The proximal 16 mm. of the stem consists of short 

 sharply annulated columnals. Distad there is a gradual transition 

 from the short ossicles to comparatively long subcylindrical colum- 

 nals. These soon reach their maximum length wliich in this species 

 is 2 mm. Continuing, the ossicles rapidly become smaller and 

 smaller, their length in some instances being a trifle greater than 

 their breadth. 



The proximal, rapidly tapering portion of the columns in these 

 forms has a remarkably large central perforation. This seems to be 

 less in the nature of an axial canal than a cavity for the reception of 

 a portion of the visceral mass. Again, it may well be that tliis cavity 

 lodged the musculature which largely controlled the movement of the 

 column. If we assume the degree of movement that seems to be 

 indicated by the structure of the columns of these forms, we must 

 assume a corresponding high development of muscles to bring about 

 such movement. Such a musculature could not be contained within 

 the narrow confines of the average axial canal. As a result of the 

 large size of the perforation, the walls are thin and this portion of 

 the stem is frequently found considerably flattened. A feature 

 which is not specially noticeable in Pleurocystis but which is strik- 

 ingly shown by Clieirocrinus is the mode of articulation of the colum- 

 nals in tliis portion of the stem. The arrangement is such that 

 great freedom of motion is possible. 



TyjJe of life led. — As indicated above, some of the members of this 

 group maintained an erect position, while others were prostrate. 

 The second type was probably derived from the first, and represents 

 a divergent off shoot comparable in mode of life to Type 3 among the 

 Cystidea, and such a form as EleutTierocrinus among the Blastoidea. 

 There is apparently no parallel case among the Crinoidea. It is to 

 be expected that the different genera referred to one of these divi- 

 sions will partake to a greater or less extent of the nature of the other. 

 A typical prostrate cystid is Pleurocystis while Glyptocystis may be 

 chosen as an example of the other division. 



Detachment among the erect, stalked Cystidea. — The evidence afforded 

 by the erect types as to the maintenance of an eleutherozoic habit is 

 as a rule inconclusive, but yet convincing. It is probable that a 



