282 DR. A. E. TBUEMAN OlST THE [vol. lxxiv, 



The suture at a diameter of 2 mm. has already a median ventral 

 saddle, and the second lateral saddle is present as a low fold near 

 the umbilical line (text-fig. 9rt, p. 281). The first denticulation of 

 the suture appears on the dorsal margin of the external saddle at a 

 diameter of 3*5 mm. A little later (after two sutures) the saddle 

 is divided into three nearly equal terminal folioles (fig. 9 b). At 

 the diameter of 8 "8 mm. the first lateral lobe is feebly trifid, while 

 the first lateral saddle has two unequal divisions (fig. 9 c). These 

 features become more distinct in the next whorl : the second lateral 

 saddle, though still undivided, is of greater size, and the first 

 auxiliary is present. 



Occurrence. — The specimens figured are from the dwdalicosta 

 sub-zone of Bracebridge (Lincoln), where similar forms also occur 

 in the Oistoceras sub-zone. It is also found in Yorkshire at a 

 similar horizon. 1 



A. crescens is distinguished by its stout form from all capricorns 

 except Liparocerns sparsicosfa, from which it differs in having 

 costse undiminished and slightly curved on the periphery and in 

 having a veiy shallow inferior lateral lobe. 



Amblycocer.vs dissottpttm, sp. nov. (PL XXIII, figs. 1 a-lc ; 

 text-fig. 10 a, p. 283.) 



Dimensions of holotype : — 



Diameter. 



Whorl height. 



Whorl thickness. 



100 mm. 



44 per cent. 



36 per cent. 



65 



45 



?44 



46 



37 



40 



A transitional form, showing Capricorn inner whorls, the outer 

 whorl expanded and bituberculate. Suture more complicated, but 

 otherwise similar to that of A. crescens. 



Development. — Capricorn ornamentation commences early ?n 

 development, fine ribs being present (20 to a whorl) up to a diameter 

 of 12 mm. From this stage the ribs are more widely spaced (14 to 

 a whorl) : such a sparsicostate stage precedes the development of 

 a bituberculate swollen whorl in several series. After the diameter 

 of 36 mm. is reached, the ribs divide into two or three on the 

 venter ; tubercles appear on the outer margin at 60 mm. diameter, 

 and the ribs are much less prominent. The inner tubercles are 

 but feebly developed. 



The arrangement of costa 1 and the depressed shape of the inner 

 whorls in this ammonite thus recall A. crescens, from which it 

 presumably is derived. 



Occurrence. — The type-specimen of A. dissotypum was col- 

 lected in the dcedalicosta sub-zone at Bracebridge, near Lincoln. 

 It is distinguished from other Liparoceratids of this stage by its 

 suture, with shallow first lateral lobe, and by its peripheral rib- 

 curve. 



1 S. S. Buckman, 'Geology of the Country between Whitby & Scarborough ' 

 Mem. Geol. Surv. 2nd ed. (1015) p. 69. 



