6 LYMAN ON A NEW STRUCTURAL 



line is axe-shaped, with a wide, slightly curved edge without, and a narrow prolongation 

 within, terminating in a small angle. Side arm-plates (figs. 5, 7), nearly meeting below, 

 widely separated above ; when stripped of skin, they present a base which has a long, 

 curved projection forward, running along the side of the tentacle-pore and ending in a 

 spine-ridge. Near tip of arm they have a wedge-shape (fig. 16), with the smaller end 

 pointing inward and downward, while the wide end is flaring and forms a spine-ridge, 

 having two or three rows of little elongated mamelons, each bearing a parasol-sj^ine. 

 Above lies a film-like upper arm-plate, of an ovoid outline with the small end forward. 

 This, as well as the side plates, is compo.sed of a translucent lime net-work, pierced with 

 numerous holes. Surrounded by these parts are the very singular arm-bones, which, 

 instead of the usual disk -like figure, are composed of two long, curved bars, lying side by 

 side and joined at each end to make the articulating surfaces, the whole forming an elon- 

 gated oval with terminal articulating surfaces. This embryonic division of the arm-bone 

 jnto its two halves continues to the disk, but the bars become proportionately shorter and 

 shorter, and the articulating surfaces approach, until, just at the mai"gin of the disk, the 

 central hole nearly or quite disappears, and the bone pretty much resembles the usual 

 type. Disk slightly puffed, and uniformly covered with minute, delicate scales, some of 

 which bear small, short spines (fig. 4). Well magnified, these scales are found to be 

 translucent and film-like, and oval in shape, not more than 0.3 m.m. long and pierced with 

 holes (fig. 9). Many of them throw up a small spine, composed of two spiculae joined 

 by cross-bars. This scaling is continued quite to the bases of the arms, for there are no 

 radial-shields. Two wide genital openings extending from outer corner of mouth-shield 

 to margin of disk. Three slender, tapering, somewhat flattened, translucent arm-spines, 

 not quite as long as two arm-joints. They are composed of two spicules, slightly serrated 

 and joined by their edges (fig. 8). At about the eighth joint the normal spines cease, 

 and are replaced by two or three crowded, irregular rows of parasol-spines or pedicellariae, 

 about twenty in all. Each of these parasol-spines is covered by a thick skin-bag, on 

 removino- which, the spine is seen to be about 1.2 m.m. long, translucent, with a regular 

 agaric top radiatingly fluted. It is mounted on a high mamelon projecting from the side 

 arm-plate (fig. 10). One short, spiuiform tentacle-scale at inner angle of lower arm-plate. 



Color in alcohol, straw. 



"Blake" Expedition of 1878-79; Station 300 off Barbadoes ; 82 fathoms; 3 speci- 

 mens. 



Ophiohelus pellucidus sp. nov. Plate I, figs. 11-15. 



Special marks. Parasol-spines short and stout ; in two rows, three or four in a row. 

 Disk smooth. No tentacle-scales. 



Description of an individual. Diameter of disk, 4 m.m. Length of arm, 12 m.m. 

 Width of arm without spines, 1 m.m. Mouth angles prominent and separated by wide 

 mouth-slits; at apex is a thick, rounded jaw-plate bearing three sharp, spiuiform tooth- 

 papillae, above which are teeth of similar shape ; on either side are two spiuiform mouth- 

 papillae, whereof the outer one is longer and serves as a tentacle-scale to the second 

 mouth-tentacle. The condition of the single specimen did not allow the forms of the 

 mouth-shields and side mouth-shields to be made out, as they were pretty closely soldered 

 aiid more or less covered with skin ; they seemed, however, similar to those of 0. wnhella 



