366 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The correspondence between the oculars of the urchins and the infrabasals of 

 the crinoids, and between the genitals of the urchins and the basals of the crinoids, 

 is thus seen to be remarkably close; in fact, the only difference between the two 

 circlets and their respective interrelationships is that in the urchins the larger 

 plates, interradially situated, exclude the smaller, radially situated, from the peri- 

 proct or apical area, while in the crinoids the larger are excluded by the smaller. 



There thus appears to be good cause for believing that the infrabasals of the 

 crinoids are the equivalent of the oculars of the urchins, and that the basals of the 

 crinoids are the equivalent of the genitals of the urchins. This second hypothesis, 

 indeed, has been almost universally accepted. 



The radials of the crinoids, usually considered the equivalent of the oculars of the 

 urchins, differ strikingly from them in (1) their indicated primarily double nature, (2) 

 their frequent separation from each other by interradial plates, (3) the fundamental 

 occurrence of plates between them and the apical portion of the animal (in addition 

 to the regularly present infrabasals), (4) in size, they being much larger than the 

 plates with which they alternate (the basals) instead of smaller, (5) in the absence 

 of plate formation under their distal border, (6) in their relation to the canals of the 

 water vascular system, which pass beyond them to the region of the infrabasals, and 

 hi (7) their relation to the muscular and nervous systems. In all of these points 

 the oculars of the urchins correspond to the infrabasals of the crinoids in so far as 

 the relationships of the latter have been determined. 



But the oculars of the urchins are always situated at the head of the series 

 of ambulacrals, while the infrabasals of the crinoids are in the later types always 

 widely separated from the radials, which form the bases of the so-called post-radial 

 series. 



The division series and the first two brachials of the free undivided arm in the 

 crinoids, the so-called interpolated series, developed in an area of skeleton-forming 

 dorsal perisome left exposed by the excess of growth of the visceral mass over that 

 of the dorsal skeleton, or rather by the much more rapid contraction of the calyx 

 plates than of the visceral mass, whereby the arm bases (the third brachials of the 

 free undivided arms) have become widely separated from the calyx plates, are the 

 equivalents of the auricles, and of the plates of the dental pyramids, in part of the 

 urchins. They were originally derived from vertical and parallel series of plates 

 resembling those in the ambulacral fields of the urchins by a complicated system of 

 segregation and fusion. The radial, being primarily double and forming the base 

 of this series, corresponds to the first two ambulacrals in the urchin to be formed, 

 that is, to the two ambulacrals situated on the border of the peristome, while the 

 subradial corresponds to all the ambulacrals of the urchin between the two situated 

 on the border of the peristome and the ocular. 



This arrangement was perfected so long ago in the phylogeny of the crinoids 

 that we get but a slight hint of it even in the earliest fossils, while in the develop- 

 ment of Antedon the interpolated series appear as a branching linear series of ossicles 

 with no suggestion of the interpolated nature of their ultimate origin. 



Apparently something occurred to stop suddenly the further development of 

 the ambulacrals in the crinoids, and the ambulacrals already formed, not being able 



