PRIMARY LARVAL PLATES OF BRACHIATE ECHINODERMS. 33 



plates of the skeleton, and has an unmistakable crinoidal facies, 

 and is highly suggestive of the singular genus Ophiopyrgus 

 amongst Ophiurids. The disk is proportionately much higher 

 than in the adult. The dorsocentral plate (fig. 16; 1) is promi- 

 nent and assumes the shape of a rounded cone. It is sur- 

 rounded by a circle of five interradially placed plates (3, 3), 

 homologous with the basals of a Crinoid, and these are suc- 

 ceeded by an outer series of five radially-placed plates (4, 4), 

 which alternate with them and are homologous with the first 

 radials of the Crinoid. 



These basal and first radial plates are of nearly equal size, 

 very tumid and almost semiglobular in form. They fit close 

 together and occupy by far the greater part of the abactinal area 

 of the disk. The terminal (ocular) plates (t, t) at the extremity 

 of the rays are large, somewhat resembling the shape of a 

 serpent's head, and are armed near the extremity with one or 

 two pairs of comparatively large robust spinelets which are 

 directed outwards. The plates of the median dorsal line are 

 large and distinct, and occupy a large portion of the abactinal 

 surface of the ray. The so-called dorso-laleral series of plates 

 form the margin of the ray, and the intermediate plates are 

 small. Between the " dorso-lateral '^ series and the adambu- 

 lacral plates there are not more than two fully developed 

 longitudinal rows of plates, with a partially developed series 

 commencing to appear between the latter and the adambulacral 

 plates. The raadreporiform body is outside and external to 

 the iuterradial plate, and almost in the ravine of the inter- 

 brachial angle. The anal aperture is excentric, situated 

 between the dorsocentral plate and an interradial (i. e. basal) 

 plate, and stands in the right posterior interradius when the 

 madreporiform body is placed in the right anterior interradius. 



In fully-grown specimens with rays 125 — 130 millim. 

 long, the plates above enumerated maintain all their character- 

 istics, and are in no way masked. They have increased greatly 

 in size, and have likewise become modified in shape. The first 

 radials 4, 4, are much larger than the terminals t, t ; and had 

 this been the form generally studied for the investigation of 



VOL. XSIV NEW SER. C 



