part 4] evolution of the lipakoceratid-E. 209 



less than whorl thickness, and a broad low-arched venter, with the 

 outer row of tubercles near the edge of the whorl. Although in 

 the type-specimen no sutures are visible, yet specimens from Napton 



and Yorkshire which appear to be identical show sutures of the 

 Liparoceras pattern (tig. 4te) intermediate in degree of complexity 

 between L. sparsicosta and L. obtusinodum. 



A complete series of specimens may be arranged, transitional 

 between L. heterogenes and L. obtusinodum. In successive 

 members at a given diameter the whorl is stouter, the ratio of the 

 numbers of outer to inner tubercles is greater, and the sutures are 

 more complex. Further, the stage at which Capricorn ornamenta- 

 tion is developed steadily becomes later, while bituberculation 

 appears earlier, so that ultimately the Capricorn stage is omitted, 

 as shown in the table : — 





I) 



iameter. 



Whorl 



Whorl 







Capricorn 





Tubercle 







mm. 



height. 



thic 



kness. 



Umbilicus. 



from 



to 



on last 









per cent. 



per 



cent. 



pei 



r cent. 



diameter 



diameter. 



whorl. 



Prom 



s 



50 



27 





33 





33 



5 



50 



Paired ., 



Napton. 



35 



29 





34 





29 



6 



19 



1 'aired. 



Warwick. 



50 



41 





52 





30 



8 



12 



23 15 



From 



( 



GO 



47 





57 





23 



— 



— 



25 15 



Battledown, 



) 



60 



48 





56 





20 



— 



— . 



23/12 



■Cheltenham. 



I 



70 



51 





61 





17 



— 



— ■ 



24 12 



Ltpakocekas obtusinodum, sp. nov. (Fig. 4 c, p. 270.) 



Dimensions of holotype (from Napton, Warwickshire) : — 



Diameter. Whorl height. Whorl thickness. Umbilicus. 



45 mm. 44 per cent. 52 per cent. 33 per cent. 



Compare Ammonites striatus Bronn. 1 



Whorl depressed, venter nearly flat, umbilicus fairly small, the 

 tubercles unpaired, about 22/18. 



The inner whorls of L. obtusinodum have no Capricorn stage, 

 but the tubercles are fine and paired until the diameter of 2-3 mm. 



L. obtusinodum occurs in Dorset in the Belemnite Stone and 

 the beds immediately below it. 2 One such form (W. D. L. Coll. 

 1493) has the following dimensions : — 



Diameter. 



Whorl height. 



Whorl thickness. 



Umbilicus. 



25 mm. 



44 per cent. 



68 per cent. 



2S per cent, 



19 



42 



63 



26 



The suture is a little more complicated than that of i. hetero- 

 genes (fig. 4 c). 



1 H. G. Bronn, ' Lethsea Geognostica ' vol. i (1835-37) p. 449 A: pi. xxiii, 

 %. 7. 



2 W. D. Lang, ' The Ibex Zone at Charmouth, &c.' Proc. Geol. Assoc. 

 •vol. xxviii (1917) p. 33. 



