82 ME. S. S. BUCKMAN OX [vol. lxxvi, 



generic position is not of importance now. What is important 

 is the recognition at, presumably, two horizons of two species 

 sufficiently alike to cause nomenclature failure and yet sufficiently 

 distinct for separation. 



If this supposition be correct, this 'Polymorpliites polymorphus ' 

 indicates a horizon about 3, and the failure of the suhpla/nicosta- 

 densinodus faunas is to be accounted for by the excavation being 

 started below their position — exposure failure. 



It is doubtful, however, whether any such explanation will account 

 for the absence of the JEchioceras-Gleviceras faunas (6, 5); and it 

 certainly will not account for the absence of the viferum fauna, 

 which should intervene between oxynotum and the Sclilotlieimice : 

 in that case, at any rate, stratal failure would appear to be the 

 explanation. 



Now the occurrence of the Sclilotlieimice at Gloucester might 

 be explained by saying that the excavation had reached a lower 

 level than in any of the railway-cuttings ; and the absence of 

 A. fowleri as due to this excavation not having gone deep enough 

 to reach the line of nodules (exposure failure), or to absence of the 

 nodule stratum (stratal failure). This theory means that the 

 SchlotJieimice occur at two horizons — in an upper horizon (clay) 

 by themselves, in a lower horizon (argillaceous limestone) asso- 

 ciated with Arietites {A. fowleri). 



Before discussing it we may consider the possibility of collec- 

 tion failure. Mr. Richardson says {op. cit. p. 154) : — 



' This clay was heaped up ... to be used for brickmaking or any other 

 purpose.' 



From clay destined for brickmaking nodules are picked out, and 

 thrown aside separately. That might possibly explain the absence 

 of any Arietites. Yet it is rare for all nodules to be picked out 

 at the first moving, and, again, an experienced geologist like 

 Mr. Richardson would not overlook the nodule heaps. Even 

 supposing overlooked nodules, th s does not necessarily disprove a 

 theory of two horizons. For, what does that theory involve ? 

 Among my father's Cheltenham specimens of Ammonites lacima- 

 tics (Sclilotlieimia spp.) there are examples from two matrices — a 

 clay matrix and a limestone matrix. If only the Sclilotlieimice of 

 the clay matrix have been found at Gloucester, and if they occupy 

 another horizon and are distinct from those of the limestone matrix, 

 then there ought not to be any of the limestone species at Glou- 

 cester, if the nodular horizon is missing. 



There are three successive waves of Sclilotlieimice in the Lias 

 (faunal repetition), not counting some species of isolated position. 

 The lowest, a series of large forms, some of them quite giants, 

 lived in late Hettangian (Hettangian 8) and finished off in early 

 Sinemurian (Lymian 1). They occur widely over Europe, from 

 the North-Eastern Alps and Wurtemberg to Dorset, Somerset, 

 and Yorkshire. 



The second wave consisted of comparatively small forms repre- 



