342 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



an infrabasiil; (4) a rirclet of five stQl larger plates each situated exactly over an 

 inl'rabasal to wliicli it is joined by a small plate intercalated in the basal ring, 

 separated from each other by the plates of the interambulacral series which follow 

 tlie basals; these, the rachals, give ris(^ on their distal border to the arms; (5) a 

 circlet of five largo approximately triangular plates witli their inner apices touch- 

 ing the peristomal area in the center of the disk, the orals. 



The specialization and perfection of the crinoid type took the form of a gradual 

 reduction in the size and complexity of the calyx, correlated with and ultimately 

 the result of, a great increase in the length and weight of the arms. The plates 

 between the iiifrabasals and the radials firet disappeared, soon followed by the 

 interambulacral series, wliich became reduced to a single plate situated between 

 the radials, this later becoming eliminated so that the radials came into contact 

 all around the calyx, forming a closed circlet like that of the basals and iiifrabasals. 



The posterior interradius, bemg of larger size than the other interradii on 

 account of the presence therein of the anal proboscis and of the posterior portion 

 of the digestive tube, was the last to be affected in the transformation from the 

 primitive more complex to the specialized simpler type of calyx, and we therefore 

 find a series of types in which only one interambulacral (interradial) plate is present 

 between the two posterior radials and only one subradial (the radianal) beneath 

 the right posterior radial. It is from this intermediate type that the yoimg of the 

 recent forms, so far as we know them, inherit their characteristics. 



The original calcareous covering of the body in the type from which the adults 

 of the recent forms inherit their characters was in the form of a globular capsule 

 composed of (1) a central plate, or centrale, usually reduphcated into a long column, 

 of which the topmost columnal is permanently attached to the apical portion of 

 the calyx; (2) a closed circlet of five small infrabasals; (3) a closed circlet of five 

 larger basals; (4) a closed circlet of radials, giving rise on their distal border to the 

 arms; (5) a circlet of five orals closing in the ventral pole. 



We see this arrangement of the calyx plates in Marsupites (fig. 565, pi. 7); 

 but in this aberrant form all the plates have adopted the same size not because 

 they are primarily of equivalent dimensions, but on account of a large increase in 

 the volume of the calyx to form a float, necessitating a corresponding increase in 

 the size of the plates which cover it. 



The essential differences between the palaeozoic crinoids (including the Encri- 

 nidje) and the later forms, stated on the basis of broad averages, arc two in num- 

 ber: (1) the column in the former is of continuous growth and of indefinite length, 

 and Li composed of undifferentiated and similar columnals, while in the latter the 

 column typically, after attaining a definite number of cohmmals, abruptly ceases its 

 growth, the topmost columnal becoming very closely attached to the calyx and 

 increasing in size, forming a so-called proximale, which is joined to the calyx by a 

 close suture and to the columnal just below it by a suture slightly less close, a so- 

 called stem syzygy; this fundamental column structure among the later forms is 

 subject to a great variety of perplexmg modifications, though it may always be 

 detected by close study; (2) the calices in the latter, which are very small, exhibit 



