ilO ME. s. s. BrcKMA]s ON [vol. Ixxviii, 



It is rather tempting to assign to the rock that date, because it 

 would bring the Thecidellce of the top of layer 7 into line with 

 Moore's zone of Tliecidium ntsticiim (p. 390). But there seem to 

 be ditticulties in this course : — (1) there is lithographic stone in 

 laj^ers 8 & 10 below; (2) there is pink rock suggestive of hifrons 

 date in layer 9 ; and (3) the possibility that the Harpoceras af£. 

 mulgravium found in a fallen block (p. 888) came from about the 

 horizon of layer 8 or even below it. 



The ammonites of layer 6 may have been derived, especially as 

 Mr. Jackson has found similar peculiar Hildoceratid ammonites 

 in far better condition nearly in the top of the bed (corresponding 

 to layer 3, or possibly 4). His specimens are in quite unworn 

 condition, showing no signs of derivation, yet it seems quite 

 impossible to imagine that they were contemporaries of Sammafo- 

 ceras — that is contrar}^ to all our experience : there is admittedly 

 a difficulty here. As to Tliecidellce, the}' can onh^ have been 

 derived if the whole slab to which they are attached had been 

 derived. This is possible ; but it is also possible that the Theci- 

 dellcB are a different species and of a different date from Thecidella 

 rustica : thej are too much weathered to make an}- determination 

 of s^^ecies satisfactory'. And, even if such were made, our know- 

 ledge of the range of Tliecidellce is admittedh' very incomplete : 

 possibly several Whitbian horizons ^deld Tliecidellce, but there has 

 been, since Moore's time, too little sj'stemafcic search for them. 

 The washing process, by which alone they can be obtained in 

 satisfactory condition, is particularly tedious ; and there are cases, 

 as their occurrence in this calcareous Watton Bed shows, where 

 it could be applied onh^ with much difficulty. 



If it be supposed that the peculiar Hildoceratids are in situ in 

 the lower part, or not much removed from their original position, 

 that the}^ were at least dejDOsited originally in a lithographic-stone 

 matrix, then we are confronted with a difficult position, and the 

 following theories may be put forward to account for the facts. 

 A recapitulation of the facts may be helpful : the}' are : — 



(1) A laminated lithographic- stone bed about .5i- feet thick shows in 



the face of Watton Cliif; separating Yeovilian from Domerian 



sandy deposits. 

 (2j It is in the same position as the Junction-Bed of Do^^ii-Thorncombe 



Cliffs ; but it is quite different in lithic character, and disagrees 



greatly in its fauna. 



(3) The "white-stone matrix runs through the bed, but there are other 



matrices mixed up with it. 



(4) Striatulvm forms occur in a yellowish matrix, but are redeposited 



in lithographic stone ; they are associated with Hammatoceras and 

 Dumortieria. 



(5) There is faunal inversion. 



(6) Certain small species of Hildoceratids of -pre-falciferum aspect are 



found at two levels, towards the base and towards the top ; those 

 at the top are in the best condition and in a white- stone matrix. 



(7) A white- stone matrix is not found in the Junction-Bed of the western 



cliffs ; but it occurs at Bothenhampton and is -post- striahdum 

 in date. 



