part 4] EVOLUTION OF THE LTPAEOCERAT1D.K. 289 



periphery, where they are very prominent. Xo longitudinal stria- 

 tions visible. Suture complex; external lobe deep and narrow, 

 with a high median saddle, external saddle high, not extending 

 quite to the outer row of tubercles, first lateral lobe a little deeper 

 than the external lobe. This species differs from P. par in odum 

 (Quenstedt) 4, pi. xxviii, figs. 16-17, in the greater arch of the 

 periphery. 



A typical specimen in the collection of Mr. Buckman (Xo. 209S), 

 from Radstock, Somerset {valdani zone), has the dimensions: 



Diameter. Whorl height. 



"Whorl thickness. Umbilicus, 



140 mm. 60 per cent. 



39 per cent. 9 per cent 



H. YlCENlXODICERAS, 



gen. nov. (See p. 264.) 



YlCIXIXODICERAS SIMPLICICOSTA, Sp. UOV. (PL XXIY, figs. 4 a 



&4 5.) 



Diameter. Whorl height. Whorl thickness. Umbilicus. 



87 mm. 51 per cent. 58 per cent. 20 per cent. 



A bituberculate sphrerocone ; whorls round, inner whorls smooth 

 (no Capricorn stage). Tubercles paired, at wide intervals on the 

 outer whorl (about twenty in each row) ; the two rows very close 

 together, the inner row being high up on the whorl. Ribs simple, 

 annular, with a faint backward curve on the periphery, not dividing 

 at the tubercles ; only about a sixth of the ribs of the outer 

 whorl bear tubercles. Except in one case the tubercles are broken 

 off ; they are longitudinally elongated, and appear to be placed on 

 one rib, touching the ribs on each side. 



Suture very complicated, with a deep external lobe and high 

 median saddle, the first lateral lobe wide and slightly deeper than 

 the external lobe ; the whole of the external saddles and part of 

 the first lateral lobe are ventral to the outer row of tubercles. 



V. simplicicosta> is distinguished by its paired tubercles from all 

 Liparoceratids, except Parinodiceras and A. striata* Reinecke, 

 from both of which it differs in having undivided ribs and the 

 tubercles placed close together; it is further distinguished by the 

 form of the whorl. 



Only three specimens of this ammonite have been examined; 

 they are in the collection of Mr. J. W. Tutcher, who obtained 

 them at Radstock, Somerset (valdani zone). 



VII. Phylogexy axd General Considerations. 

 Early Capricorn Forms. 



Although the capricorns of the Liparoceratidse are most abundant 

 in the davcei zone, yet more primitive capricorns have been found 

 as low as the oxynotum and raricostatum zones. Two such form- 



