78 BULLETIN 84, UNITED STATES NATIONAL MUSEUM. 



offers distinct plates, each of which carries a rather elongated little stump, which 

 is thick, cylindrical, having a rough surface and its end terminated by a few 

 elongated spinules which are rather thin and somewhat divergent. One may 

 generally see ten protruding and more or less visible radial ribs at the end of each 

 of which there is a Uttle radial triangular shield, which is bare and separated from 

 the one opposite by a rather narrow interval. On the larger specimens, the 

 diameter of the disk of wliich ranges from 8 to 10 or 11 mm., the outhnes of the 

 upper plates become less discernible, though stiU distinct, and the Uttle stumps 

 which they carry are shorter and much stronger; the spinules at the ends of the said 

 stumps are also fewer, shorter, thicker, unequal, and rather erect. The radial 

 ribs are not always apparent, but one can easily see the very small radial sliields, 

 which are lying fairly close. In the largest specimen which I have studied (station 

 2668), the diameter of the disk of which reaches 12 mm., the little staves of the upper 

 face of the disk appear in the form of thick, elongated granules with rough ends, which 

 at first sight differ from those of the smaller specimens in wliich the diameter of the 

 disk does not exceed 5 mm., but which are, nevertheless, connected with the latter 

 through a set of intermediate forms. 



Like arrangements are found again on the under face of the disk in the inter- 

 radial spaces. The stumps of the plates extend up to the mouth shields and 

 become progressively smaller and shorter, and at the same time, less rough and less 

 spinulous. 



The shape of the mouth sliields varies with age. In the smaller specimens 

 (plate 15, fig. 5), these sliields are in the shape of triangles or lozenges, a little wider 

 than long, with an obtuse proximal angle and a coffvex distal side, spUtting some- 

 times into two sides united by a very plainly rounded angle. As the Ophiuran is 

 growing, the mouth shield becomes longer and it very soon grows to be longer 

 than wide; it then appears fairly narrow and rather small, wider va its proximal 

 part than in its distal region, mth an exceedingly obtuse proximal angle which 

 may even reach 180°, converging lateral sides and a very narrow and rounded 

 distal side (fig. 4). This shape reminds one, as already pointed out by Sars, of 

 that of the mouth shields of 0. spectaiilis. The adoral plates, which are middle- 

 sized, are short, fairly broad, and have parallel sides. The oral plates are high and 

 triangular. Sars, in his description, indicates four oral papillae on each side. In 

 fact, I never fiad more than three in the young ones; they are thin, very long, 

 conical, pointed, and have about all the same shape although their thickness 

 slightly increases from the external to the internal papiUa. There is an odd dental 

 papUla which is stronger than the neighboring papillae, elongated and conical. In 

 the adult, the number of oral papiUae is increased by the intercalation of two, or 

 even three, supplementary papillae, which break the regular arrangement of the 

 first papillae with which they are not in line; the result is that the oral papillae 

 show some variations in number and disposition. Generally speaking, the external 

 papiUa always remains somewhat wider than the others. Moreover, near the 

 till then single dental papilla, are seen one or two other papillae, so that the jaw 

 is terminated with dental papillae, the number of wliich varies from one to tliree. 

 Consequently, the specimens which had first presented, by the arrangement of their 

 oral and dental papillae, the characters of the true Ophiacantha in the restricted 



