136 COLLECTIONS FKOM MELANESIA. 



4. OpMopinax stellatus. (Plate VIII. fig. C.) 

 Pectinuva stellata, Lyman, Prel. List, p. 3. 



Disk somewhat pentagonal, not puffed, with a central rosette of 

 plates varying a good deal in the extent to which it. is well defined, 

 and occupying about half the upper surface ; in some cases a central 

 plate and two not very regular circlets of variously sized plates of 

 irregular form can he made out. The interradial series of plates, 

 which extend from the rosette to the edge of the disk, are stouter 

 and more prominent than the radial series; tiny are composed of 

 three or four plates varying in size and form, and sometimes paired 

 at the margin of the disk ; the larger median have smaller plates on 

 either side. The radial series may, for its undivided portion, be 

 formed by a single plate, but, as a more general rule, there are 

 three ; there are no smaller marginal plates. The bifurcated band 

 which embraces the base of the arm has usually three pieces on 

 either half. Just below the margin of the disk there is a prominent 

 plate, which is placed interradially ; in shape this is irregularly 

 cordiform ; above, on either side, it is bounded by a compressed or 

 diamond-shaped plate ; between it and the mouth-plate there is a 

 pair of smaller plates and several still smaller plates intervening ; 

 on the side there is an elongated plate, with two smaller ones, lying 

 along the edge of the genital slit. The radial shields are large and 

 triangular, and fill up all the space on the disk between the radial 

 and interradial plates which is not occupied by the rosette. 



The inner mouth-shields have a straight outer edge, the sides are 

 at first straight, and then bending inwards meet one another at a 

 more or less acute angle ; they are longer than broad ; the space 

 between them and the edges of the mouth is rather loosely granular ; 

 the outer mouth-shields are about as broad as long, and have a 

 slightly convex outer edge ; the umbo is well marked, and that 

 shield is not divided ; the side mouth- shields are well developed, 

 and are separated from the genital slits by a small granulated patch. 

 Seven mouth-papilla;, the three innermost small, the innermost 

 smallest ; the next three large, increasing in size from within out- 

 wards ; the outermost again small. 



The arms begin to narrow immediately after leaving the disk, 

 and become very delicate at their free ends : in a well-developed 

 specimen, in which the disk measures 17 miliim. in diameter, they 

 are 70 miliim. long, 5 miliim. wide, and 4-5 miliim. high at their 

 insertion into the disk. Upper surface carinated ; upper arm-plates 

 about three times as wide as they are long, but, owing to the invasion 

 of the side arm-plates, the aboral is longer than the adoral edge ; 

 both these edges arc straight. 



There is a pair of pores between the first and second lower arm- 

 plates ; the lower, like the upper, arm-plates have their aboral longer 

 than their adoral edges, and this, as on the upper surface, is clue to 

 the encroachments of the side arm-plates, and is more strongly 

 marked near to, than far from, the disk. None of the edges of these 



