216 BULLETIN 82, UNITED STATES NATIONAL MUSEUM, 



and the calyx plates, it is necessary here to include an account of the later develop- 

 ment of this organ. This has been carefully worked out by W. B. Carpenter; he 

 writes: "Concurrently with the advance in the development of the calj-x (see 

 beyond under Development), the column undergoes an increase both in the number 

 and in the length of its component segments, and while it also increases to some 

 extent in diameter, its solidity is still more augmented by the endogenous growth 

 of its calcareous skeleton. The terminal stem plate augments both in diameter and 

 in thickness, absorbing into itself, as it were, nearly the whole of the organic sub- 

 stance of the basal disk. Its typical form may be considered as circidar, but its 

 margin is usually more or less deeply divided into lobes. Its diameter is usually 

 about 0.015 inch. In its center is a deep depression that lodges the end of the 

 lowest columnal. The length of each of the original columnals is augmented by new 

 calcareous deposits at the extremities which finallv become compactly rounded off 

 and well defined, so that the apposed surfaces of two segments are clearly marked 

 off from each other instead of having their irregularities commingled as in the 

 earlier period of their formation. The diameter of each segment increases by new 

 calcareous deposit on its cylindrical surface, bringing up its whole length to the 

 size of the first formed median ring and finally giving to its extremities a slight 

 excess beyond this. At the same time the solidity of each segment is increased by 

 an inward extension of the calcareous trellis-work which progressively fills up what 

 was at first a hollow cylinder. This internal solidification, however, goes on more 

 slowly than the completion of the external form and dimensions of the segments, 

 for these may present their mature aspect, or nearly so, while possessing so little 

 substance that their shape is materially altered by the drj-ing up of the soft sarcodic 

 axis of their interior. While the original segments are thus advancing toward 

 completion, new segments are being developed in the interval between the highest 

 of these and the base of the calj-x. Bj' the time that the opening out of the cahTC 

 commences the number of columnals has usually risen to 15 or 16, those of the 

 inferior third of the column are pretty nearly solidified throughout, but those of the 

 middle and upper thirds are still so far from having attained their completion that 

 their calcareous cylinders when broken across are found to be mere shells. The 

 highest plate, upon which the base of the catyx rests, is now distinguished from those 

 below it by its somewhat larger diameter, but it does not as yet present any approach 

 to the peculiar shape which it afterwards comes to possess. Tlie entire column 

 remains clothed with a thin layer of sarcodic substance and its cavity is occupied 

 by a cylinder of the same which forms a continuous axis throughout its entire 

 length and passes up at its summit into the cal3'x." 



Carpenter was unable to find at this stage any traces of that fibrous structure 

 which may be distinguished about the ends of the segments at a subsequent time. 



He continues: "During the earher part of the spreading out of the caly^;, a 

 continued increase takes place in the number of columnals by the development of 

 new rings at its summit, while the previously formed columnals of its middle and 

 upper portions become progressively elongated and soUdified as those of the lower 

 portion have previously been. At or about the period at wliich the change takes 

 place in the relative positions of the oral and anal plates, the production of new 



