PLATYLEPAS DECORATA. 429 



rostral end rounded, rather narrower than the other end, and curled 

 a little inwards. Terga, of nearly the same shape and nearly as long 

 as the scuta: the carinal end is rather more pointed than the scutal 

 end of the valve, and when viewed internally, the growing surface of 

 this end is seen to be bluntly pointed. In both valves the upper layers 

 of shell usually scale off. 



Mouth and Cirri. The only differences, compared with the follow- 

 ing species, that I perceived, were that the hairs at the end of the palpi 

 were here rather longer, and the fourth tooth in the mandibles rather 

 less distinct. The rami of the first cirrus are very unequal in length. 



2. Platylepas decorata. PL 17, fig. 2 a — 2 b. 



Shell with fine longitudinal ridges, ornamented in the 

 lower part by minute beads ; parietes not porose : basal 

 membrane equalling in convexity the shell. 



Hub. — Pacific Ocean ; Galapagos Archipelago ; Lord Hood's Island, Low 

 Archipelago ; Mus. Cuming, Darwin. 



General Appearance and Structure. — Shell oval, with the orifice large. 

 The walls are thick, aud of less height from top to base than in the last 

 species. The surface (and this is the chief external character) is 

 marked by fine longitudinal ridges, each of which, when closely ex- 

 amined, is found to be double (fig. 2 b), and near the basis, where not 

 abraded, to be ornamented with minute beads on each side: these 

 beads are largest on those ridges which border the sutures. The 

 parietes are not permeated by pores ; on their inner surfaces there are 

 a few rather prominent but short ridges, instead of the many finer 

 lidges, as in P. bissexlobata. The sheath is of great thickness ; in 

 young specimens layers scale off its summit, all round the shell, as in 

 Tubicinella ; it is only slightly hollowed out at its lower margin ; it 

 descends more than half-way down the walls. The midribs, formed 

 by the folded walls, are more prominent than in the last species, and 

 descend lower down ; hence the basal membrane is more convex than 

 in the last species, for it projects downwards as much as the shell pro- 

 jects upwards. In medium-sized specimens, the midribs project inwards 

 to a distance exceeding half the breadth of a compartment. A less 

 proportional length of the midrib, in an inward direction, is formed by 

 the inward folding of the wall, and a greater length by the thickening 

 of its inner longitudinal margin, than in the last species. The basal 

 edge of the inner end of the midrib is smooth. 



Dimensions. — The average size of the numerous specimens which I 

 obtained at the Galapagos Islands was about *2 of an inch in diameter, 

 and I found none larger; but Mr. Cuming's specimens from Lord 

 Hood's Island are half an inch in diameter. 



Opercular Valves. — These closely resemble those of the former 



