158 JURASSIC BIVALVIA AND GASTROPODA 



tubercles are present where the ribs are crossed by spiral threads. Base evenly 

 convex, ornamented only with strong spiral threads. Last whorl tending to become 

 slightly disjunct, deviated, and narrower in cross-section just before the aperture. 

 Aperture almost circular, but extending obliquely below the foot of the columella, so 

 that the inner lip is situated well to left of axis of shell. 



Holotype and paratypes. Holotype, no. GG.10295, ex B.P. Coll. About 50 

 paratypes (mostly imperfect) including nos. GG. 10294, GG. 10296-97. 



Localities and horizon. Mandawa well no. 6, Tanganyika, at depths 50-52 

 feet, 52-54 feet, 54-56 feet, 56-58 feet, 58-60 feet, 66-68 feet, 68-70 feet, 70-72 feet. 

 Near site of Mandawa well no. 1. All Bajocian (?). 



Remarks. This species could be described as an Exelissa with the ornament of a 

 Rhabdocolpus. It is probable that Exelissa is a polyphyletic genus, its various spe- 

 cies, characterised by the disjunct and deviated condition of the last whorl just before 

 it reaches the aperture, having been derived independently from various groups of 

 Procerithium. The taxonomic implications of this theory cannot, however, be ex- 

 plored here. After careful study of the series of specimens now described, the con- 

 clusion has been reached that they all belong to the same species, although the outline 

 of the whole shell and of individual whorls varies considerably and the Exelissa con- 

 dition of the apertural region of the last whorl seems to be more marked in some 

 specimens than in others. 



Exelissa dodsoni sp. nov. 

 PI. 26, figs. la, b 



Specific name. After Mr. R. G. Dodson, of the Geological Survey of Kenya, 

 collector of the type-specimens. 



Diagnosis. Shell of medium size for the genus (usual height 6-7 mm.), conical 

 or slightly cyrtoconoid, spire angle about 15 °. Protoconch elevated, acute, but 

 eroded in available specimens, so that the number of whorls forming it is uncertain. 

 Teleoconch whorls with flat outer face and broad, well impressed sutural region. 

 Ornament consisting of collabral ribs overridden by spiral threads. Collabral ribs 

 on most whorls narrow but prominent, orthocline or slightly prosocline, separated by 

 slightly broader intervals, and extending right across the outer face of the whorl, their 

 upper end projecting at the impressed sutural region ; they number about 12 on the 

 later whorls on which they are developed. On the later whorls, however, especially 

 the last, the lower half of each rib, or even the whole rib, tends to be only faintly 

 developed. Spiral threads 6-7 in number on the spire whorls, where they are mostly 

 unequal in strength and extend on to the sutural depression. Small granules may be 

 present at the intersections of the spiral and collabral elements, particularly on the 

 thread forming the lower border of the sutural depression. Last whorl tending to 

 become narrower in cross-section and slightly disjunct just before the aperture. 

 Base evenly convex, ornamented with spiral threads. Aperture oval, broader than 

 high, extending obliquely below the foot of the columella, so that the inner lip is 

 situated well to the left of the axis of the shell. 



