176 



PAL/KONTOLOGY 



One of these is the Oppelidae (III.), keeled forms 

 which appear suddenly in the oxycone stage, without 

 sign 'of preceding stages, and mostly remain in that stage 

 or tend to open out into scaphitoids ; they tend to produce 

 well-marked lappets and a rostrum, so that their radial 

 line is sickle-shaped (Figs. 46, d; 50) ; their suture-line has 

 a smooth outline and a very prominent first-lateral saddle 

 and deep first-lateral lobe (Fig. 48, c). 



The other is the family Stepheoceratidae (II.), using 



FlG. 50. LUNULOCERAS BRIGHTI (PRATT), CALLOVIAN (LOWER 



OXFORD CLAY), CHRISTIAN MALFORD (WILTS). 



(Natural size.) 



Rostrum and lappets. Original. 



that term in its widest sense : a very extensive group, 

 which starts from a coronate stage, and goes through 

 various developments of form some becoming planulates 

 directly, others sphserocones (Fig. 46, k), others planulates 

 through a sphaerocone stage, but all retaining the same 

 general type of ribbing more or less straight ribs, 

 splitting on the lateral area into two or more which 

 either run continuously across the arched periphery 

 (Fig. 46, h) or are interrupted by a groove (Fig. 46, *'), 

 never a keel. The aperture may have a plain border, 



