MONOGRAPH OF THE EXISTING CKINOIDS. 313 



Infrabasals. 



In the crinoids the infrabasals normally form a closed circlet of five small 

 plates about the dorsal apex of the animal, resting with the inner portion of their 

 external faces upon the topmost columnal (figs. 570, 571, pi. 7). 



The infrabasals, which correspond to the oculars in the echinoids, are inter- 

 somatic in position, each being situated directly beneath a radial; they alternate 

 with the larger basals, which, forming a similar closed circlet just beyond them, 

 are midsomatic in position and correspond to the echinoid genitals. 



The infrabasals are the first plates in the intersomatic or radial series, and are 

 the only true calyx plates belonging to that series, the radials and following ossicles 

 being, strictly speaking, brachials. 



Ordinarily the plates succeeding the infrabasals are arranged uniserially, at 

 least for a short distance; but in the genera PromacTiocrinus and Tlwumatocrinus 

 (figs. 113, 114, p. 181) each infrabasal is followed by two radials instead of by the 

 usual one so that the arrangement here is in certain respects homologous to that 

 which is found in those echinoids which possess multicolumnar ambulacral series. 



There appears to be a definite connection and correlation between the infra- 

 basals (and the oculars, which correspond to them in the echinoids) and the suc- 

 ceeding series of plates, just as there is a definite correlation between the basals and 

 the orals, though of entirely different significance. 



In the urchins the oculars always stand at the head of the ambulacral series, 

 from which they are never separated. In certain crinoids a subradial plate occurs 

 between the basals beneath the right posterior radial which connect the infra- 

 basals and the radials, representing the entire ambulacral series of the urchins 

 except for the plates immediately surrounding the peristome, which correspond to 

 the radials. This, however, is an exceedingly rare condition. 



While in the echinoids the oculars always remain extremely important con- 

 stituents of the test, and are perhaps the most important plates of the coronal ring, 

 the general tendency in the crinoids has been toward the suppression of their 

 equivalents, the infrabasals, and with the suppression of the infrabasals has come 

 the similar suppression of the following series of plates which are usually, and 

 always in the later types, dispensed with altogether except for the radials, repre- 

 senting the echinoid ambulacrals immediately surrounding the peristome, and these 

 are now separated from the infrabasals by a closed circlet of basals. 



In the blastoids the conditions are essentially similar to those in certain crinoids; 

 there are no infrabasals, and the ambulacral or radial series is reduced to the forked 

 plate, corresponding to the radial, which encloses the ambulacrals, corresponding 

 to the brachials of the crinoid arm. 



In the crinoids the infrabasals lie at the distal end of the radial water tube, in 

 exactly the same position as the oculars are found in the echinoids. The water 

 tube of the arms is in reality merely a side branch from the true water tube, which 

 runs around the side of the body from the circumoral ring to the infrabasals, and 

 has no further morphological significance. Though in the later crinoids the water 

 tube leading from the edge of the disk to the infrabasals is insignificant when com- 



79146 Bull. 8215 21 



