LIOCERAS AMBIGUUM. 27 



chamber the ventral area is convex, scarcely defined, and has an indistinct keel. This 

 keel is at first formed by the junction of the two convex portions of the ventral 

 area, continuing to meet at a gradually less and less angle, until towards the end of 

 the body-chamber the ventral area is very nearly rounded. Inner margin very shal- 

 low, much sloped on the body-chamber and concave, sometimes slightly crenulated 

 by the ends of the small radii in the interior. The amount of inclusion exhibited at 

 the mouth in the type is a trifle less than half a whorl. (In the figure a little too 

 much inclusion is shown here.) At a whorl farther back, we find that nearly the 

 whole of the preceding whorl is occluded. (A trifle too much of this whorl is shown 

 in the figure.) Previous to this we find that the amount of inner whorls exposed 

 is proportionately more, and they show us that in youth the specimen was 

 possessed of ribs. The mouth-border is seen in fig. 3, taken from a slightly larger 

 specimen, which exhibits its sigmoidal shape very well. Fig. 4 gives a view from 

 above showing the rounded end of the ventral lappet, but this figure omits to give 

 a very slight keel, which is just perceptible along the middle of the ventral area. 



A larger specimen, probably adult, measuring 6| inches in diameter, shows the 

 same characters, but the coiling of the umbilicus is more regular, so that the amount 

 of the inner whorls exposed is rather greater, and the appearance flatter, and the 

 recession of the whorl at the body-chamber is not so abrupt. 



Young Shell (figs. 5, 6). — Compressed, subcarinate, whorls nearly parallel, 

 slightly concave along the inner area. Up to the diameter of about 10 lines the 

 whorls are evidently ornamented with ribs, probably V-sbaped, and sometimes 

 uniting close to the inner margin ; beyond that size the ribs gradually give place to 

 similar lines of growth, with occasional undulations in the shell. The ventral area is 

 broad compared with the other parts of the aperture, and flattened, with a small, just 

 distinct carina. The inner margin is not defined until the ribs disappear, then it 

 becomes concave. The umbilicus is wide and open on account of the presence of the 

 whole of the body-chamber with its receding inner margin. (The actual mouth-border 

 has been slightly broken.) The inclusion is about half a whorl, but evidently, to 

 conform to the dimensions of the umbilicus of the type, the inner margin must for a 

 whorl back advance towards the centre (supposing the shell to continue its growth), 

 and envelop nearly the whole whorl as the additional air-chambers are formed. 1 



This is a species which, when well preserved, may readily be separated from the 

 others, but a comparison of poorly-preserved specimens makes the task very 

 difficult. Fortunately most of the specimens of this and the other species are in 

 good preservation, but especially the specimens figured on Plate VII, figs. 1, 2, 5, 6. 



1 This shows that alterations took place in the air-chambers and in the position of the inner 

 margin, and that when the air-chambers had been partitioned off they were not unalterably fixed, 

 but still had to undergo certain developments and modifications. The whole genus Lioceras gives us 

 examples of this, but perhaps this species more than any other. 



