158 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Now tho effect of the nioveniciits of the posterior end of the digestive tube 

 upon tho progressive reduction in the size of the calyx, and upon the reduction of 

 the jiuniber of the calj-x plates, is continually to hijider its progress in the posterior 

 interradial or anal area, so that this area constantly reniabis somewhat larger than 

 the others and is the last one from which the primitive calyx plates, having become 

 functionlcss and obsolete, are dropjjcd. The lateral and ventral movements in the 

 posterior end of the digestive tube cause a continual liftmg stress, wliich is exerted 

 in a diagonal direction toward the upper right-hand corner of the posterior inter- 

 radial or anal area, or more correctly result in pro])pmg up this corner of the pos- 

 terior interradial area, as well as tho right posterior postradial series, so as greatly 

 to liinder the consummaticm of the reductive processes. 



As a consequence of this force, alwa3's present and constantly exerted, the 

 interradial and other plates in the posterior interradial area are able to maintain 

 their hidividuality and their existence long after they have entirely disappeared 

 from all the other areas, while as a result of the constant propphig up of the right 

 posterior ray the subradial plate is able to maintain itself under that ray long 

 after it has disappeared from beneath all of the others; at the same time the tend- 

 ency to reduction, which is just as strong in the posterior as in the other inter- 

 radial areas, wiU be confhied to the left-hand side of that area, so that all of the 

 plates and structures lying in it will be distorted and turned toward the right. 



The presence of the persistent subradial plate under the right posterior radial 

 is a characteristic feature of many genera in the Flexibilia, and, so far as is known, 

 this plate is always present in the young of the recent forms (fig. 563, pi. 6). But 

 its true significance and its homologies have heretofore never been understood; ia 

 tho fossil types it has been considered a distinct entity and dignified by tho name 

 of radianal, while in the recent types, as for instance in Antedon, it has always been 

 known as the anal, though it has nothing whatever to do with the so-called anal 

 of the fossil species. 



Tho observed tendencies in the species of the fossil Crinoidea Flexibilia, and 

 tho effects which we would naturally infer would foUow in crmoids undergoing 

 reduction in the size of tho visceral mass and of the calj'x wliich possess a digestive 

 tube of the typo occurring in the recent species (excepting certain comasterids) for 

 purely mechanical reasons, are thus seen to be in perfect agreement. 



As the entire test of tho urchin, except for its small apical portion, is com])aral)lo 

 to that ])art of the crinoid between the apical system and the arm bases, it naturally 

 follows that any increase in the i)lates of the latter in this intermediate area is a 

 step in the direction of the urcliins. 



The radial is the equivalent of two of tho ambulacrals of the urchins; the 

 radianal (or any one of the sul)radials) is the counterpart of another (single) ambu- 

 lacra! formed between the radial, wliich represents tho two radial aml)ulacrals border- 

 ing the peristome, and the infrabasal, which represents the ocular. 



Thus tho subradials of tho crinoids are formed exactly in the same place and 

 in the same manner as the series of ami)ulacrals in the echuioids, and they not onlj' 

 give us a valuable clew to the paths of divergence of the crinoids and of the echinoids 



