364 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



surrounding the periproctal area with five small oculars situated between their 

 outer angles, this arrangement giving a maximum of rigidity. 



Now the oculars of the echinoids are most intimately associated with the 

 series of ambulacrals, and the genitals are associated with the interambulacral 

 series. Therefore in any readjustment by which five of these plates came into 

 mutual contact, excluding the other five from contact with the periproctal area, 

 each of the 10 plates must maintain its original association with the series of plates 

 arising from it. 



As the genitals are much larger than the oculars, such association can only 

 be maintained by the exclusion of the oculars from the original circlet, for the 

 exclusion of the larger genitals by the sudden growth of the oculars behind them 

 would mean the more or less serious constriction, or at least crowding, of the 

 series of ambulacrals. 



In the crinoids we find indicated as a primitive condition for the class a, closed 

 ring of five small infrabasals just beyond which is a second closed ring of five much 

 larger basals which alternate with them; the former are radial in position, the 

 latter interradial. Beyond the basals is a third ring, sometimes closed and some- 

 times partially or entirely open, of radials, alternating with the basals, and hence 

 in line with the infrabasals. These radials are each primarily double plates, and 

 moreover they belong morphologically with the series of brachials and are not 

 properly calyx plates at all; they do not always form a closed ring, for they may 

 have one or five interradials intercalated between them, and furthermore they may 

 be separated from the basals, or from the infrabasals below them, by one or more 

 subradials. 



The mechanical conditions affecting the crinoid calyx are very different from 

 those affecting the echinoid test. The fixation by means of a stalk imposes a 

 very considerable strain upon the apical plates, which therefore are at once obliged 

 to adjust themselves to a position and mutual interrelationship of the maximum 

 rigidity. 



In the echinoids the original circlet of plates about the periproct becomes 

 reduced from 10, 5 large alternating with 5 small, to 5 composed of the larger 

 only, the smaller becoming excluded and accommodated between the distal angles 

 of the larger. 



The crinoid calyx commences with a circlet of five small plates, radial in 

 position, just beyond which is a circlet of five larger plates, interradial in position; 

 all the plates of both circlets are usually in mutual apposition. It occasionally 

 happens, however, that the smaller plates are somewhat separated so that the 

 larger reach the summit of the column between them, and we find an apical sys- 

 tem composed of five large (interradial) and five small (radial) plates alternating, 

 exactly as in the echinoids, except that the larger plates are in contact beyond the 

 smaller ones. 



The small plates of the first circlet in the crinoids (infrabasals) are radial in 

 position, exactly as are the small plates (oculars) in the coronal system of the 

 echinoids, and in both classes the large plates (basals and genitals) are situated 

 in the interradii. 



