part 4] 



EVOLUTION OF THE LIPABOCERAT1 D.K. 



285 



iameter. 



Whorl height. 



Whorl thickness 



mm. 



per cent. 



per cent. 



0-8 



40 



73 



1-5 



43 



64 



3-5 



42 



56 



8-0 



42 



42 



16-0 



36 



37 



28-0 



28 



28 



The shell is usually very thick, and the sutural development is 

 therefore difficult to study in well-preserved specimens such as 

 those found at Waddington. The sutural development resembles 

 that of other capricorns in that : 



(1) The first suture has a narrow, undivided, median saddle (fig. 11a). 



(2) The second suture has a shallow external lobe without a siphonal 



saddle (fig. 11 b). 



(3) A minute siphonal saddle has appeared in the external lobe, which 



is deeper than the lateral lobes, by the fourth suture (fig. lie). 



(4) The first denticulation appears on the dorsal side of the external 



saddle. 



Occurrence. — Oistoceras sub-zone, Yorkshire and Lincolnshire. 



Oistoceeas FiGULiisruM (Simpson). (Fig. 12 b.) 

 Eefigured, 1, vol. i, No. 26 a. 

 An advance on Oistoceras omissum ; the number of ribs on 



Fig. 12. — Oistoceras allceotypum («) and O. figulinum (b) ; 

 both from Lincoln. 



a 

 90 mm. 



b 



40 mm 



• \ 



[a = Specimen figured in PI. XXV, fig. 1.] 



a whorl increases in early development, but the ribs of the last 

 whorl are more widely spaced. 



Occurrence. — Oistoceras sub-zone, Yorkshire, Lincolnshire, 



? Dorset. 



