VESTINAUTILUS PAUCICARINATUS. 87 



small central vacuity. The whorl is trapezoidal in transverse section ; in a more 

 detailed view it is seen to be decagonal, — that is, it is made up of ten surfaces and 

 angles. The peripheral area is broadly arched in general outline, but it presents, 

 when more closely examined, three zones, the centre of which may be slightly 

 concave ; this is bordered by the two lateral zones, which descend rapidly to the 

 umbilical margin. The umbilical declivity is again divided into two tolerably 

 distinct areas (at least until the aperture is nearly reached) ; the upper, which is 

 much the narrower, being the space marked off by the inner and outer keels of 

 the shell; the lower comprising the space between the inner keel and the line of 

 junction of the whorls. The remaining area is the impressed zone which, in the 

 present species, is divided into three rather obscurely defined zones corresponding 

 with those of the peripheral area which it embraces iu the young shell. The 

 overlapping or inclusion of the whorls extends to the umbilical marginal keel in 

 the young shell, but in the adult it sometimes, though not always, leaves this keel 

 free for the last half-whorl. The umbilical slopes are very slightly inflated below 

 the inner keel ; the whorl in the proximity of the body-chamber becoming also 

 distinctly rounded on the umbilical shoulders, owing to the dying off of the keels 

 bordering and just inside the umbilicus. The principal keels are those here 

 referred to, the marginal one being the strongest, and resembling, in every respect, 

 the one occupying the same position in V. cariniferus. It borders the umbilicus, 

 which it completely encircles, dying off gradually as the body-chamber is reached. 

 The inner keel is much less prominent than the outer one, to which it is 

 approximate and parallel ; it generally extends to within a very short distance 

 from the aperture. The peripheral are generally much less conspicuous and 

 prominent than the umbilical keels, and in some species they almost entirely 

 disappear in the adult shell. Typically there are three on each side of the more 

 or less flattened central zone of the peripheral area, the inner one being much less 

 distinct than the outer ones. In the specimen figured (PI. XXIV, fig. 1 b) these 

 ventral keels are remarkably well developed. They are, in general, equidistant, 

 the distance of the outer one from the umbilical keel being slightly greater than 

 that separating them from each other. They are generally to be seen faintly on 

 casts (PI. XXIV, fig. 2). 



The septa in this species are approximate, their distance from each other 

 varying from 6*5 mm. to 8*5 mm. measured on a cast in the median line of the 

 peripheral area (PI. XXIV, fig. 2), the width of which is here from 50 mm. to 55 

 mm. The sutures have a curvature on the umbilical slopes of which the concavity 

 is directed anteriorly. On the peripheral area they make the figure of a bent bow, 

 the principal curvature of which has its concavity or sinus directed towards the 

 aperture, as is always the case. There is an annular lobe (Little Island specimen). 



