MUSEUM OF COMPAEATIVE ZOOLOGY. 255 



imbricated scales, -whereof the lower ones are nearly as large as the mouth- 

 shields, near which are a few grains. Radial shields small and not touching ; 

 nearly twice as wide as long, and separated from upper surface of arm by a 

 cluster of scales. Genital openings extending from mouth-shield to margin 

 of disk. Two stout, cylindrical, blunt arm-spines, about two thirds as long as 

 a joint. One large circular tentacle-scale. Color in alcohol, pale gray. 



Station 239; Grenadines, 383 fathoms, 6 specimens. Station 260, Grenada, 

 291 fathoms, 5 specimens. 



The only other species of this genus, 0. epigrus, comes from Low Archipelago, 

 southeast of the Sandwich Islands, at a depth of over 2500 fathoms. ■ 



Ophiocoma pumilla Ltk. 

 Bartlett, Pedro Bank, 4 fathoms. 



Ophiochiton ternispinus sp. nov. 



Plate V. Figs. 67-69. 



Special Marks. — One tentacle-scale. Three slender, rather long arm-spines. 



Description of an Individual. — Diameter of disk 12 mm. Length of arm 

 60 mm. Width of arm near disk 2 mm. Twelve mouth-papillae to each 

 angle, whereof the four inner ones, on either side, are short, pointed and spaced, 

 while the two outer ones are flattened and crowded. Teetli stout, flat, and 

 shaped like a blunt spear-head. Mouth-shields regular pointed heart-shaped, 

 with a small rounded lobe without. Length to breadth, 2 : 1.3. Side mouth- 

 shields very narrow within, where they barely meet ; but spreading widely 

 without, where they bound a part of the inner end of the genital opening. 

 First under arm-plate rounded and small, not larger than the neighboring 

 mouth-papilla. The plates beyond are much narrower than the arm ; they 

 are about as broad as long, much wider without than within, with an outer 

 curve, re-entering curves on the sides, and a truncated angle within. They 

 are somewhat swollen, but have no ridge. Side arm-plates even, slightly Bar- 

 ing, nearly meeting above and below. Upper arm-plates narrower than the 

 arm, much longer than wide, bounded on all sides by gentle carres. Disk 

 covered with thin, irregular, overlapping scales, and haying in the centre a 

 group of much larger rounded primary plates, 1 mm. in diameter. Below, 

 the scaling is similar but finer. Radial shields narrow oblong, or egg-shape, 

 small, separated by a narrow wedge of fine scales ; length to breadth, 2 : 1. 

 Genital openings large and long, extending from month-shield to margin of 

 disk. Three smooth, slender, tapering arm-spines, whereof the appermost 

 may be as long as two joints. Lengths, to that of an under arm-plate, 

 2.5, 1.2, 1.2 : 1. One oval tentacle-scale of moderate size. Color in alcohol, 

 nearly white. 



"Porcupine" Expedition, 1869, Station 42, southwest of Ireland. B6S fath- 

 oms, water 4°.3 Cent., 1 specimen. 



