SIBOGLINUM ROBUSTUM 255 



section, and a short way in front of it runs the transverse groove dividing the 

 protosoma from the mesosoma. This is strongly marked on the ventral side 

 and notched in the mid-ventral line, where the tip of a lappet points forwards 

 (Fig. 1275), while the groove dies out towards the mid-dorsal line. Behind 

 the point of attachment of the tentacle is another transverse groove. The 

 comparatively large protosoma tapers towards the front end (Fig. \21Л, B) 

 and the tentacle, the greater part of which has not been preserved, is attached 

 in the mid-line. No pinnules are present on the part of the tentacle which 

 remains. The conical cephalic lobe is not large. The mesosoma is three times 

 as long as broad, and narrows slightly behind the bridle. It then broadens out 

 again, only to taper once more towards the hind end, which is curved at the 

 sides and just perceptibly concave on the ventral side (Fig. 121 A, B). The 

 delicate keels of the well-developed bridle, lying on the front edge of low 

 cutaneous ridges, are not fused ventrally, and the gap between their dorsal 

 ends is very wide (Fig. \21A). The ventral surface of the mesosoma has a 

 shallow median furrow. 



The front section of the metasoma has pronounced lateral ridges bounding 

 a rather narrow ventral sulcus, and the large metameric papillae have hazy 

 outlines (Fig. 127 B). The broad dorsal ciliated band begins at the front edge 

 of the metasoma. The region of the thickened papillae is unknown. There are 

 three girdles, the first two lying some way in front of the third. The first is 

 interrupted on the dorsal side, the other two on the ventral side, and their 

 well-defined ridges bear uniserial toothed platelets (Fig. \21E) of charac- 

 teristic shape. Each platelet is divided by a transverse line into two parts, 

 the front part smaller and narrower, with teeth pointing backwards, and the 

 hind part larger and broader, with teeth pointing forwards. In profile the 

 platelets appear very slim, and the central zone can be seen to lack teeth. 

 The ratio between the sizes of the front and back parts varies : sometimes the 

 front part is only a little less than half the area of the platelet, but for the 

 most part it occupies about a third of it (Fig. \21F). Most of the platelets are 

 17-18ju long, exceptionally 16 or 19^; their breadth in the middle is с. 5/г. 



Since the only specimen is incomplete it is not possible to determine more 

 than a few of the dimensions of the body: length of the fore-part of the body, 

 a little more than 0-9 mm, breadth, 0-2 mm; the metasoma is the same 

 breadth; in the region of the girdles the trunk reaches ОТ 5 mm in diameter; 

 the distance between the second and third girdles, 0-3 mm. 



The distinctive, thin-walled, soft transparent tube possesses very regular 

 muddy-brown rings, whose length is about a quarter the diameter of the 

 tube, while the completely colourless, glassily transparent interspaces between 



