FROM TANGANYIKA AND KENYA 157 



elevated, acute, of about z\ smooth whorls. Succeeding whorls with fiat to feebly 

 concave outer face and broad, angularly impressed sutural region. Ornament con- 

 sisting of collabral ribs which extend right across the whorls except for the impressed 

 sutural region, and are overridden by spiral threads. Collabral ribs narrow but 

 moderately strong, straight or opisthocyrt to a varying extent, numbering 10-12 on 

 the later whorls, and separated by intervals about twice their width. Spiral threads 

 on spire whorls most commonly 5, more rarely 6 or even more, one forming each 

 border of the sutural depression ; in some specimens they are almost equal in 

 strength and evenly distributed, while in others they are unequal, alternating in 

 strength or quite irregularly arranged. Small granules situated at intersections of 

 collabral ribs and spiral threads are present on uppermost and lowest of the latter in 

 most specimens, but in a few specimens on uppermost only, and in some on inter- 

 mediate threads also ; strongest granules pointed, majority rounded. Base evenly 

 convex, ornamented with strong spiral threads. Aperture (broken in most speci- 

 mens) apparently evenly rounded. 



Holotype and paratypes. Holotype, no. GG. 10290, ex B.P. Coll. Many 

 paratypes, including nos. GG. 10291-93. 



Localities and horizons. Mandawa well no. 6, Tanganyika, at the following 

 depths : 38-40 feet, 44-46 feet, 46-48 feet (very common), 48-50 feet, 50-52 feet 

 (common), 52-54 feet, 56-58 feet, 58-60 feet, 60-62 feet, 62-64 feet, 64-66 feet, 66- 

 68 feet, 68-70 feet. Mandawa well no. 7, depth 92-100 feet. Bajocian (?). 



Remarks. The muricate type of ornament present in this species is found in both 

 the subgenera Rhabdocolpus and Xystrella, although in the more typical species of the 

 latter such ornament is more coarsely developed. According to Cossmann the most 

 important distinction between the two subgenera is that the aperture is rounded in 

 Rhabdocolpus and quadrangular in Xystrella ; hence, if this criterion is accepted, the 

 species now described must be included in Rhabdocolpus. Its ornament differs in 

 detail from that of any described European species. 



Genus EXELISSA Piette i860 



Exelissa africana sp. nov. 

 PL 27, figs. 2a, b, 3a, b, 4a, b, $a, b 



Diagnosis. Of medium size for the subgenus (height of largest specimens about 

 8 mm.), conical to cyrtoconoid, acute, mean spire angle most commonly about 10 °. 

 Protoconch finely pointed. Whorls high, tending to be loosely coiled, with flat to 

 feebly convex outer face and wide, well impressed sutural region. Ornament consist- 

 ing of collabral ribs overridden by spiral threads. Collabral ribs confined in many 

 specimens to adapical half of whorls and strongest near the suture, but stretching in 

 some specimens from suture to suture, prosocline, straight to rather strongly opistho- 

 cyrt ; their number, breadth and distance of spacing varying in different specimens. 

 Spiral threads 8-10 or more on later whorls, subequal to distinctly unequal, present 

 on the entire surface, although least conspicuous near the adapical suture when the 

 collabral ribs there are strong. When the collabral ribs are well developed small 



