LIOCERAS AMBIGUUM. 29 



Lioceeas ambiguum, var. cosTATTJM, S. Buchnan. Plate VII, fig. 7. 



Discoidal, compressed, sub-carinate, whorls almost flat, ornamented with 

 sigmoidal ribs, which are not much bent on the lateral area, and are rather irregular 

 in size, especially on the body-chamber, where they really become undulations. 

 Ventral area narrow and sloping, with a small, barely distinct carina. On the body- 

 chamber the ventral area becomes broader and flatter ; and, the carina almost dis- 

 appearing, the ventral area on the end of the body-chamber appears nearly rounded, 

 while the lines of growth, which were formerly curved forwards on it, now pass over 

 it nearly at right angles. Inner margin sloping and concave. Inclusion covers 

 nearly the whole whorl except where the body-chamber is present, when it suddenly 

 decreases to half the whorl, as is shown in the specimen figured. The mouth- 

 border, of which a portion is shown on the figured specimen, is probably like that 

 in Lioceras ambiguum. 



This form differs from the true Lioceras ambiguum in having a smaller, deeper 

 umbilicus, a less compressed inner area, and well-marked ribs, which last are most 

 conspicuous on the middle of the lateral area, and on the body- chamber gradually 

 become more like waves. (The irregularity of the ribs, depressions, and fine 

 sigmoidal lines of growth are hardly sufficiently brought out m the figure.) Like 

 Lioc. ambiguum it possesses a somewhat inflated, slightly gibbous, outer and 

 ventral area near the end of the body-chamber ; but, not having such a marked 

 depression in the inner area, it does not show this character so conspicuously. 

 From Lioc. bradfordense this form is best separated by its irregular ribs. They do 

 not actually bifurcate, but between them intermediate ribs occur on the outer por- 

 tion of the whorl. The ribs, too, are more conspicuous on the middle of the lateral 

 area, whilst those of Lioc. bradfordense are stronger on the outer area. The shape 

 of the umbilicus also differs, being more occluded, especially in the inner whorls. 

 The whole shell is also thinner than Lioc. bradfordense ; and in all its proportions 

 except the size of its umbilicus, and in not having quite so much depression, it 

 closely agrees with Lioc. ambiguum. 



I have every reason to believe that the Murchisonce-zone is the correct horizon 

 in which this variety is found. The figured specimen has not its locality recorded, 

 but I judge it to be from Haselbury in Somerset. From that locality I have some 

 other very closely allied forms, not well preserved, which differ slightly in their ribbing 

 and the coiling of the umbilicus, but not materially in their general appearance. 



In all these specimens the suture-line differs slightly from the sutures of Lioceras 

 in general in having a deeper superior lateral saddle (a singular fact when we 

 consider that such usually accompanies ribs that are much produced on the lateral 



