30 INFERIOR OOLITE AMMONITES. 



area), a shorter inferior lateral lobe, and a rather greater distance between the 

 sutures. 1 Haselbury is the only locality where I know this form in the South- 

 west, and it is evidently very scarce ; but I have obtained, from Birdlip, in Glouces- 

 tershire, from a fallen block of Oolite belonging probably to the Sandy Ferruginous 

 beds, 2 a portion of a specimen which has every appearance of being this form. 



Plate VII, fig. 7, shows a well-preserved specimen of Lioceras costatum, with 

 its test complete and the commencement of the mouth-border. The absence of a 

 piece of test on the other side shows us the sutures. 



Lioceras decipiens, S. Buchnan. Plate XII, figs. 8, 9. 



Discoidal, much compressed, carinate, whorls very slightly convex, entirely 

 without any ornamentation except sigmoidal lines of growth. Ventral area very 

 narrow, only slightly sloped, with a carina somewhat distinct except on the body- 

 chamber. Inner margin sloping, concave, and shallow. Inclusion normally about 

 three-fourths of the whorl, slightly less when the body-chamber is present. Umbilicus 

 small, with a constant increase of the amount exposed, and with few turns because 

 the diameter of the shell increases quickly on account of the breadth of the whorls. 



It is possible to confound Lioceras decipiens with almost any of the other species 

 of this genus ; but if the points of divergence be accurately noted and the deter- 

 mination of the other species be correctly made, it will be seen that this one differs 

 in many points, which, though individually small, become collectively important. 

 From any varieties of Lioceras concavum with which I am acquainted it is separable 

 by the entire absence of ribs, by its larger umbilicus, which continually exposes a 

 greater amount of the inner whorls, by the flatness, but not actual concavity, of 

 the inner area, and by the more actual separation of the carina from a flatter 

 ventral area. It has a peculiar umbilicus, smaller than that of Lioc. bradf or dense, 

 and, when the body-chamber is absent, even smaller than that of Lioc. giganteum ; 

 but when the body-chamber is present the umbilicus expands suddenly to about 

 the same size as the umbilicus of the latter without its body-chamber, so that this 

 species can be readily distinguished by its method of coiling. It is also smoother 

 at all ages, much thinner and flatter, with more parallel sides, broader whorls, 

 a narrower and flatter ventral area, and therefore seemingly a more important 

 carina. The last, too, distinguishes it from Lioc. opalinum, which has a much 

 sloping ventral area without a distinct carina, and has not such a sudden expansion 



1 My present intention is to figure the suture-lines of the various species together in convenien'- 

 groups for the sake of comparison, and then to discuss them. 



2 See se.-tion of Haresfield Hill (p. 43). The beds described as D' are probably on the horizon 

 from which this specimen came. 



