34 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



Isocrinus wyville-thomsoni (PI. 5, fig. 13), seem to be more or less 

 characteristic of the Pentacrinidje as a whole. In some cases it is to 

 be noted that the deposit of stereom smoothly rounds off the entire 

 exposed face of the distal ossicle and forms an appreciable knob. I 

 In other specimens a smaU plug is formed which merely closes the 

 axial canal. In these individuals the radial striae are at times still 

 to be seen. It may be that these differences in the structure repre- 

 sent stages in the attainment of freedom, or it may be that in different 

 species there is a certain degree of variation in this regard. The 

 essential feature after all is the plugging of the axial canal. Subse- 

 quent deposition of calcareous matter seems merely somewhat to 

 protect the distal nodal from fracture. 



The method of detachment as it obtained among these forms is 

 open to some question. Thomson (1869-72, p. 768) says: "There 

 can be no doubt that in early Ufe the Crinoid is attached and that it 

 becomes disengaged by the witheiing of the lower part of the stem." 

 Carpenter (1884, p. 22) says: "The free mode of hfe appears to be 

 attained in these individuals, not by actual fi'acture of the stem at 

 a node so as to shorten it more or less, but by the lower and therefore 

 older part remaining undeveloped, while new joints appear in suc- 

 cession above it, each growing to a larger size than those previously 

 formed. The stem thus becomes slender and tapering and but ill 

 adapted for attaching itself below; but its length is not diminished 

 so much as if it were broken at a node." 



These conclusions have been based upon the fact that in many 

 specimens there is a decided diminution in the size of the column in 

 its distal portion. At times this decrease seems to be effected through 

 the gradual tapering of the stem. In other cases the column tapers 

 somewhat distad, and then decreases abruptly in diameter from nodal 

 to nodal. As has been pointed out by Carpenter (1884), such varia- 

 tion in the diameter of the stem is far from being a uniform char- 

 acter. It is significant that adult specimens apparentl}^ do not show 

 this structure. 



On the whole, from the evidence available one would judge that 

 a gradual tapering of the column such as is figured by Carpenter and 

 reproduced here (PI. 4, fig. 2) in the case of Isocrinus decorus is such 

 as is due to normal growth. In cases where the column suddenly 

 becomes smaller distad from a given point this explanation does not 

 seem wholly adequate. It would appear that in such a case, after ■ 

 the detachment of the organism, a certain portion of the column ceases 

 to enlarge. 



Both Thomson and Carpenter, as quoted above, take a stand 

 against the actual disruption of the column, but it is inconceivable 

 that detachment could have occurred except as the result of such a 

 process. A "withering" or nondevelopment of the distal portion of 



I 



