18 THE VOYAGE OF H.M.S. CHALLENGER 



Gould ; Ischnochiton diverge/is, Reeve ; Lepidopleurus proteus, Eeeve ; Lepidopleurus 

 ustulatus, Eeeve. The first has the central areas uniformly shagreened, and the lateral 

 areas with fine ridges. The remaining forms have the jugum of the central areas sha- 

 greened and the pleura finely ribbed, the lateral areas being distinct and having coarse, 

 interrupted, and irregular ridges. The extreme of this style of sculpture is reached in 

 Callistochiton antiquus, Reeve, with its strong pleural ridges and secondary cross ridges 

 on the central areas, and the two prominent coarsely rugose ridges on the lateral areas. 



The colour of the half-dozen specimens is greyish, yellowish, or rusty, mottled and 

 passing into a deeper colour at the edge, forming a distinct brownish, greenish, or orange 

 border to the shell ; median dorsal line with a series of dark spots. Ligament more or 

 less banded. 



Lepidopleurus, Carpenter. 

 Lepidopleums dorsuosus, n. sp. (PI. I. fig. 5, PI. III. figs. 5a-5i). 



Station 145a. December 27, 1873. Lat. 46° 41' S., long. 38° 10' W. Prince Edward's 

 Island. 310 fathoms. Volcanic sand. 



Shell. — Remarkably compressed laterally, the two sides meeting at an angle of 75°. 



Anterior valve. — Concentrically rugose, with about twenty-six radiating low irregular 

 ridges, some of which bifurcate. 



Intermediate valves. — Central area: with juguin and anterior border striated, the 

 pleura with distinct horizontal ridges. Lateral areas : distinct, concentrically rugose, 

 usually with three main radiating low ridges, the anterior and posterior of which are 

 often bifid. 



Posterior valve. — Concentrically rugose, with fifteen irregularly arranged low radiat- 

 ing ridges. The anterior ridge is broader than the others, and may be slightly bifid. 

 Umbo rudimentary ; anterior border smooth, with slight concentric lines of growth. 

 Lamiuse of insertion moderate in size. 



Girdle. — Narrow. Upper surface with ovoid, slightly overlapping, somewhat granular 

 scales ; edge with minute projecting spicules under surface, with long, narrow, closely 

 applied scales. 



Colour. — White. 



Size. — Length, 21 "5 mm.; breadth, 7 '5 mm.; height, 6 mm. 



Gills. — Extending to more than half the length of the foot, twenty in number, the 

 middle somewhat larger than the remainder. 



The name dorsuosus, full of ridges, is applicable to this species on account of its 

 numerous longitudinal and radiating ridges, and at the same time recalling the steep 

 ridge of the shell itself. 



