70 THE VOYAGE OF H.M.S. CHALLENGER. 



extremity, and gapes somewhat at both ends and along the back. It is dirty whitish, 

 concentrically coarsely plicate -wrinkled, the rugse being stronger upon the central 

 portion of the valves, becoming more or less obsolete towards the dorsal margin on both 

 sides. The surface or outer shell-layer is everywhere rough to the touch, minutely 

 granular, and produced in the left or flat valve beyond the white shelly interior, forming 

 a broadish buff border composed of a compact tessellation of thousands of the minutest 

 plates as it were soldered together. The dorsal margin is slightly arcuate on the 

 anterior side and somewhat oblique, posteriorly being almost rectilinear, and horizontal in 

 the right or deep valve, but a trifle oltlique and concave near the beak in the left. In 

 this valve, on both sides of the umbo, it is turned over abruptly, almost at right angles 

 towards the other valve. The lower outline is straightish along the middle, consideraljly 

 up-curving in front, and only a little behind, forming with the oblique latter margin a 

 well-rounded corner. The umbones are small, very acute, not at all inflated, and turn 

 towards the hinder end. On this side the dorsal area is excavated at the beaks, where it 

 is rather sharply margined on each side. The external ligament extends on both sides of 

 the umbones, in front having a narrow short groove partitioned ofl' for its reception. 

 The internal cartilage is large, equilaterally triangular, located in a strong shelly process 

 firmly soldered to the valves immediately beneath the beaks, which is unusual. The 

 muscular scars are high up, the posterior being nearly circular, near the dorsal margin, 

 and somewhat remote from the end of the shell. The anterior is in about the same 

 position, elongate, narrow, bent in the middle. The siphonal inflection of the jjaUial line 

 is deep, broad, rounded at the end, and extends across the valves about five-fourteenths 

 of the entire length of the shell. 



Length 58 mm., height 32, diameter 10. 



Habitat. — Station 162, oft' East Moncoeur Island, Bass Strait ; in 38 to 40 

 fathoms ; sand and shells. 



This fine large species is remarkable on account of its flattened compressed char- 

 acter, and being almost equilateral. The ligament-pit is very strong, and the outer 

 epidermal shell-layer is peculiar. I have much pleasure in naming it after my friend 

 the Rev. R. Boog Watson, who is carefully preparing a Report upon the Gasteropoda 

 and Scaphopoda of the Expedition. 



Thracia myodoroides, n. sp. (PI. VI. figs. 6— 6&). 



Testa subcompressa, sequilateralis, aliquanto triangularis, postice late truncata, antice 

 rotunde cuneata, albida, minutissime granulata, incrementi lineis hie illic inconsjiicuis 

 arata. Margo dorsalis anticus valde obliquus, rectiusculus, vix excurvatus, posticus 

 leviter declivis, rectilinearis. Yentris margo parum arcuatus, antice lente adseendens, cum 

 dorsali acuto-rotundatim junctus. Umbones parvi, acuti, latus versus posticum vix 



