The Marine Fauna of the Mergui Archipelago. 167 



(14) Ophiopteron gymnatum, nov. sp. (Figs. 1 and 2.) 



The disc-diameter is 5 mm. and the length of the arms is 25 mm. 



The disc is round, and is covered with plates without any spines on the 

 dorsal surface. There is a large centro-dorsal shield surrounded by two 

 circles of smaller irregularly-shaped scales. The radial shields are large, 

 triangular, twice as long as they are broad, almost touching without where 

 there is a small notch for the insertion of the arms, and slightly diverging 

 inwards where they are very pointed. They are separated from each other, 

 except at the margin of the disc, by two or three elongated over-lapping 

 scales, which increase in breadth inwards. The interradial space is covered 

 by about three irregular rows of somewhat elongated imbricated scales, 

 the centre row being larger than the side ones which bound the radial 

 shields. A distinct purple line, which stretches along the upper side of 

 each arm, extends inward to the centre of the disc along the scales 

 separating the radial shields. 



On the interbrachial space, on the ventral side of the disc, no plates 

 are visible, but the space is filled, with the exception of a narrow border 

 along the genital openings, with some twenty to thirty short cylinders, 

 quite separate from each other. These cylinders with their smooth shaft, 

 terminated in a crown of. three diverging short prongs, recall the disc-spines 

 of Ophiopteron sibogae Koehler. There is a very distinct genital plate 

 bounding the large genital openings. 



The oral shields are fairly large, broader than long, roughly three-sided, 

 the outer margin gently rounded with a large genital process, and the two 

 inner sides slightly curved and meeting at a not very sharp point within. 

 The adoral plates are narrow and meet internally. The jaw plates have 

 a small triangular notch between them, and bear two regular lateral rows 

 of teeth and an irregular middle row. 



The dorsal arm-plates are large, broader than long, angular and six-sided, 

 with the distal margin only slightly broader than the proximal one. Of 

 the other sides, the two adjacent to the proximal side are longer than the 

 two adjacent to the distal side, and hence a dorsal arm-plate is broadest 

 about one-third of the length from its distal margin. 



The ventral arm- plates are also large. The innermost one is nearly 

 square, but along the arm they become rectangular, with the length 

 greater than the breadth. There is a linear division between the plates, 

 and there is no notch in either distal or proximal border. Two purple lines, 

 one on each lateral margin of the ventral arm -plates, extend along the whole 

 length of the arms. 



