PHARYNX OF AN UNKNOWN HOLOTHURIAN. 257 



plates " pharyngealia," to distinguish them from those forming 

 the actual anterior ring '^ annularia." The radial pharyngealia 

 are of a nearly uniform size and of hexagonal form. They 

 are disposed in each radial interval^ with an alternate arrange- 

 ment so that the lateral prominent angle of each plate on one 

 side of the double row fits into the receding angle formed by 

 the junction of two adjacent plates on the other. In the 

 radial series no accessory ossicles are present. The radial 

 pharyngealia are not quite flat, but somewhat concave on their 

 internal faces, and on their external faces show traces of slight 

 ridge-like thickenings directed radially from their centres 

 towards their angles. They are stout and of considerable 

 thickness, and show microscopically a structure composed of a 

 very close network of short rounded bars of semitransparent 

 calcareous matter firmly fused together. Towards the posterior 

 extremity of the pharynx in the specimen, beyond the points 

 at which the iuterambulacral pharyngealia cease, the radial 

 pharyngealia become narrowed and elongate antero-posteriorly, 

 and curved towards the cavity of the pharynx so as to form 

 semicanals. These enclose what appear to be the remains of the 

 tentacular canals. Part of this curling may be due to the fact that 

 the membrane connecting the pharyngealia is here torn away. 

 Whilst the radial pharyngealia show a tendency to be elon- 

 gated longitudinally, the iuterradial are elongated transversely. 

 They are similarly disposed in double rows. Their outer 

 margins are closely applied to those of the radial, and their 

 inner united together much in the same way as those of the 

 radial. In the two lateral interradial series, however, irregular 

 small ossicles are interposed in the median intervals between 

 the two rows of plates for part of their lengths. The interradial 

 pharyngealia do not extend so far posteriorly as the radial, and 

 also are not, like the radial, equal in length in all the series. 

 The two ventral series are longest, the single dorsal next in 

 length, and the two lateral shortest, but the extents shown in 

 the figure are not quite certain, owing to the imperfect condition 

 of the specimen. The interradial pharyngealia are much less 

 solid and compact than the radial, and readily become broken 



