parti] JURASSIC CHRONOLOOr: LEAS. 85 



heautotype from clay of Gloucester has ribs which show a 

 distinct flexure. This point need not be laboured : it is only 

 mentioned as something to be on guard about in the future ; 

 •especially as an almost identical straight-ribbed form is figured by 

 ■Greyer from Hierlatz. 1 



Another Hierlatz species, Schlotheimia geyeri, has much of 

 the character of Schl. parva. Two other species, Schl. angusti- 

 sulcata Geyer, Schl. sulcata (Simpson), are closely allied to Schl. 

 subpolita : the first being recorded from Hierlatz and Yorkshire. 



These species have much in common, and form what, may be 

 called the snbpolita group, fairly separable from the lacunata 

 group. But, if the characters of the subpolita group indicate 

 those forms which lived during denotatus time, why are they 

 absent from the Beta-Kalk of Wurtemberg, seeing that they 

 range beyond that country ? It is a legitimate point for enquiry, 

 and the first speculation which arises is this — Are there three 

 horizons ? thus : 



(a) lacunata, 



(b) subjiolita, 



(c) denotatus. 



I am not claiming that there are ; but attention must be called 

 to this possibility, for the purpose of precision in records. It 

 is safer to work with too many than with too few horizons for 

 recording purposes : thus, it is better to keep the subpolita and 

 denotatus records separate, where the one occurs without the 

 other. 



The surmise of three horizons would involve two suppositions: — 



(1) That the Wiirtemberg strata are deficient in something — show a non- 

 sequence — between the Beta-Kalk and the lacunatus clays ; (2) that the 

 association at Cheltenham of subpolita in nodules with Arietites foivleri is 

 due to paucity of sedimentation, which has made what should be a sequence 

 look like contemporaneity of deposit. 



If these suppositions could be substantiated, and they are put 

 forward in the hope that work in the field, especially in York- 

 shire, may succeed in yielding evidence, then there would be 

 a possible explanation of the absence of the subpolita fauna from 

 the Beta-Kalk of Wurtemberg and the absence of Arietites of the 

 denotatus pattern from Hierlatz and Spezia (Italy). 



On the basis of three horizons this position would be reached 

 as regards the faunas of Cheltenham and Gloucester. The former 

 shows faunas of all three presumed horizons : a clay with the 

 species of the lacunatus group, a nodular limestone— a condensed 

 deposit — containing the subpolita and Arietites faunas, in juxta- 

 position. The Gloucester deposit, on the other hand, has yielded 

 only the fauna of the lacunatus group, and has not revealed the 

 faunas of the nodular horizon because there has been collection 

 failure, exposure failure, or stratal failure. 



1 VIII, pi. iii, fig. 21, Mgoceras sp. ind. 



