78 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 41. 



while several centro-dorsals of Antedon from the Upper Chalk are almost equally- 

 gigantic. 



The massive arms of many of the Comatulse seem to mark the 

 animals as far from primitive, and certainly not as ancestral to 

 modern types. In certain cases a biserial arrangement of the arm 

 ossicles is had, such as for instance in Antedon thiollieri figured by 

 de Loriol (1884-1889, PL 224), A similar form, possibly congeneric^ 

 has been described and figured by Jaekel (1891) as Pachyantedon 

 heyrichi. The lack of any recorded tendency toward the acquisition 

 of similar types of arm structure elsewhere among the Comatulse 

 seems to indicate that these crinoids form a distinct off-shoot from 

 the comatulid stock, if indeed it does not indicate a separate origin 

 for them. Bather (1900, p. 195), states that "there is also traceable 

 in the arms a gradual attenuation * * * " when considering the 

 evolution of the Antedonidse. The differences to be observed are 

 not evolutionary in the sense that there is a gradual diminution in 

 the thickness of the arms, but rather points to a polyphyletic origin 

 of the forms involved, or at least a considerable divergence within 

 the stock. 



Again, among most of the fossil Comatulse as is shown by figures 

 3, 4, 7, 8, Plate 8, the basals are well developed and appear promi- 

 nently on the exterior of the theca. Among recent forms the basals 

 are greatly reduced (PI. 5, figs. 10 to 12) and seldom are to be seen 

 externally. In many cases this reduction in size may be held to be 

 the result of a gradual evolutionary process subsequent to the assump- 

 tion of an elutherozoic habit. On the other hand, as is shown by 

 TMolliericrinus (PL 7, figs. 3 and 4), the basals have become obsolete 

 in the adult stages, as regards their appearance on the outer surface 

 of the cup. This elimination became effective while the stock was 

 yet attached or but irregularly eleutherozoic. 



The axial canal traversing the central-dorsal of a comatulid is as 

 a rule closed and smoothed over by a secondary deposit of stereom. 

 In many cases, however, the lumen is open at the distal face of the 

 centro-dorsal, so that its nature may be ascertained. In TMolliericri- 

 nus (PL 7, fig. 5), the perforation is circular, or somewhat oval, corres- 

 sponding to the shape of the stem-lumen in the Bourgueticrkiidse 

 in general. In many cases, as, for instance, in certain species figured 

 by Carpenter (1880& and 1881), the central perforation of the centro- 

 dorsal is pentagonal to sharply stellate in section. How much reli- 

 ance can be placed upon these features as determining independence 

 of derivation it is hard to say on account of the insufficient data 

 at hand. It appears, however, that they are of some value. In 

 this connection should be noted the observation of Carpenter that 

 the centro-dorsal of Antedon has a stellate lumen when the animal 

 becom.es detached from the column. This is somewhat remarkable 



