13-2 THE VOYAGE OF H.M.S. CHALLENGER. 



The Dorsal Tubercle is broadly cordate, with both horns rolled inwards. 



In external appearance this species is not unlike Microcosmus polymorphus, Heller, 

 but it is scarcely so rough and irregular in shape. The apertures are both distinct and 

 prominent (PI. XIV. fig. 1), especially the branchial, and the test in their neighbour- 

 hood is considerably thickened and corrugated, elsewhere it is rather thin. The mantle 

 is strong, and muscle bands are greatly developed on the right side and on the dorsal region 

 of the left. In the branchial siphon, above (anterior to) the circlet of tentacles, four large 

 bluntly conical processes project from the lower edge of the sphincter muscle into the 

 lumen of the tube, and form a sort of imperfect diaphragm. 



Another point in which this species differs from Microcosmus polymorphic, is the 

 branchial sac, which has only six folds on each side. The folds are rather narrow compared 

 with the interspaces (PL XIV. fig. 2). The transverse vessels are strong, and are usually 

 alternately larger and smaller (the latter are very frequently traversed by the stigmata). 

 Sometimes, however, there are three sizes occurring alternately (PL XIV. fig. 4, tr., tr.' 

 and tr."). Besides this, about every fifteenth vessel is enormously enlarged (PL XIV. 

 fig. 3, tr.x.), and forms a broad band traversing the sac. 



There are eight or nine internal longitudinal bars on the upper side of the fold, so that 

 the figure (PL XIV. fig. 4, br.f.), only shows half of the fold. The meshes in the inter- 

 space between the folds are of two distinct sizes, and there are six of each kind. The 

 larger ones are overlapped by the fold, and about three of them are usually hidden 

 (PL XIV. fig. 2). 



One specimen of this species w r as obtained in Torres Strait, between Australia 

 and New Guinea, at Station 188 ; September 10, 1874 ; lat. 9° 59' S. ; long. 139° 42' E. ; 

 depth, 28 fathoms ; bottom muddy. 



Microcosrmis propinquus, Herdman (PL XIV. figs. 5 and 6). 



Microcosmus propitiquus, Herdinan, Prelim. Rep., Proc. Roy. Soc. Edin., 1880-81, p. 55. 



External Appearance. — The body is oblong-ovate or almost triangular in shape, and is 

 compressed laterally. It is attached by the posterior two-fifths of the ventral edge. The 

 anterior end is narrow, and terminates in the branchial projection ; the dorsal and ventral 

 edges slope backwards to the broad and rounded posterior end. The branchial aperture 

 is terminal, and is situated on a large projection turned ventrally and slightly to the left 

 side. The atrial aperture is also prominent ; it is on the dorsal edge, three-fourths of the 

 way from the anterior to the posterior end, and is directed dorsally. 



The surface is wrinkled and minutely grooved, but not covered with excrescences ; it is 

 somewhat corrugated around the apertures, and has a few foreign bodies adhering. 



The colour is pale yellow, with a reddish-brown tinge here and there. 



Length of the body, 7 "5 cm.; breadth of the body, 5 cm. 



