1926 SYSTEMATIC 29 



Oct. 



Sands : at Hartwell the clay (or clays) with the same species underlies 

 the Lydite Bed. 



Further objections to the Survey contention are that no species 

 of Hartwell-Clay ammonoids have been produced from the rotundum 

 clays of Chapmans Pool, from the Swindon Clay, nor from the Littleworth 

 Lydite Clay of Wheatley and Shotover : these clays yield Lydistratites 

 and Pallasiceras, which have not yet been produced from the Hartwell 

 Clay, but have been produced from the Lydite Bed of Hartwell and 

 Long Crendon (T.A. CCCLIII, DCV, DCVl). Then the isochronism of 

 the Thame Sands, of Thame and Long Crendon, with a part of the 

 Shotover Grit Sands is attested by the species of Paravirgatites (T.A. 



cccvin, cccLxxxn, dciii). 



An interesting observation may be made about the Paravirgatitan 

 strata (Shotover Sands) of Shotover. Working from south to north 

 there are three exposures seemingly on the same general level — two in 

 the brickyard and one beyond the waterworks. These exposures yield 

 different ammonoids and are presumably distinct in date, there being 

 distinctions also in lithic character of the sands and of the contained 

 doggers. On the south is the earUest with WhecUleyites, in the middle 

 come Pectinatites, Keratinites and Shotoverites, while from the northern 

 pit came Paravirgatites. It is only Paravirgatites which has yet been 

 found in the Thame Sands to the eastward : one specimen was in 

 place and came from sandrock near the top (T.A. DCIII). 



The true thickness of the Shotover Sands of Shotover may, therefore, 

 be much greater than the apparent thickness — there may be obhque 

 bedding. 



Reference has been made above (p. 25) to the difference, in so-called 

 Aposphinctoceras-species, in regard to the length of the primary ribs : 

 those of Hartwell-Clay species being short — the division into secondaries 

 taking place about the middle of the lateral area ; those of the Littleworth 

 Lydite-Clay species being long — the division taking place near the ventral 

 border. There is a certain altemance in date in respect of showing this 

 character, as the following Table illustrates. 



In regard to long and short primary ribs, a warning should be given 

 about side-view figures reproduced from photographs. If a short-focus 

 lens be used, the length of the primary rib in relation to the secondary 

 rib is enlarged, because the true length of the secondary rib is unduly 

 reduced. It is easy to see why ; but it is a point that may be overlooked 

 in comparing figures. Also, care has to be taken in judging the 



