314 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



pared with that of the arms, in the earlier forms, in which the calyx was very large 

 and the arms very short, the latter must have been very insignificant when com- 

 pared with the former. 



In studying the homologies of the echinoid and crinoid plates in the developing 

 young we are at a great disadvantage; for in the young crinoid the infrabasals are 

 so atrophied as largely to have lost any fundamental significance which they may 

 originally have had ; the plates (theoretically) normally present between the infra- 

 basals and the radials do not appear at all, except for the right posterior which is 

 formed, very late in life, far out of its normal position; and the basals have become 

 enormously enlarged, composing the entire dorsal investment of the calyx and, being 

 in mutual apposition, widely separating the infrabasals from the succeeding plates in 

 the radial series. 



As I understand it, it is the atrophy of the infrabasals, the suppression of the 

 plates between the infrabasals and the radials, and the enormous growth of the 

 basals which have combined to exclude the infrabasals from their primitive posi- 

 tion and primitive connection with the distal end of the water tube. 



But it should be emphasized that the water tube grows not only outward into 

 the arm (an offshoot of purely secondary morphological importance) but downward 

 into the centrodorsal; in other words, it eventually comes into its true relations with 

 the infrabasals by growing beyond the radials. 



In the later fossil and in the recent crinoids the infrabasals are greatly reduced 

 and functionless, or absent altogether; but as the structure of the animals by the 

 application of the well known law of Wachsmuth and Springer is shown to be 

 dicyclic it is assumed that they are either present in the young, but become resorbed 

 during the ontogeny, or that they have so recently disappeared that their effect 

 upon the general structure still persists. 



In many of the later fossil and in the recent crinoids (excepting those of the 

 family Plicatocrinidas) the column is characterized by a definite growth limit after 

 reaching which no further development occurs, but the topmost columnal enlarges 

 and becomes permanently attached to the calyx by close suture, forming a so-called 

 proximale which is in all essentials an apical calyx plate. With this proximale the 

 infrabasals, greatly reduced and concealed by the column, fuse, forming with it 

 what is practically a single ossicle. This condition occurs in all the recent coma- 

 tulids in which infrabasals have been observed, the centrodorsal being formed 

 partly by the greatly enlarged topmost columnal, now become an apical calyx plate, 

 and partly by the circlet of infrabasals fused with it. 



In the two pelagic comatulids, Marsupites and Uintacrinus, we find, as would 

 be expected, an aberrant partial reversion to primitive conditions resulting from 

 the absence of a column and the consequent absence of the factors which call for 

 a great reduction in size of the calyx plates and for their coalition into a compact 

 mass. In Marsupites, which is an extreme type, the five infrabasals are of enor- 

 mous size (fig. 565, pi. 7), as large as the basals and the central apical plate, and 

 form a very important part of the calcareous investment of the body. The enor- 

 mously elongated arms of Uintacrinus necessitated a great reduction in the size of 

 the plates covering the body, though in this genus we frequently, but not always, 



