454 ME. s. s. BUCKMAN ON" [vol. Ixxviii, 



A last word may be added concerning the Japanese species 

 Dactylioceras lielianthoides Yokoyama. Dr. Spath quotes this 

 from above the Fish-Bed : I quote something like it from well 

 below. The Dactjdiocerates are a ver^^ difficult group, occurring 

 in the Upper Lias in seemingly endless Yariety. When it was 

 thought that the Upper Lias was laid down at about three dif- 

 ferent dates, such endless variet}" of apparently contemporaneous 

 forms was difficult to understand. When it is seen that the dating 

 of the Upper Lias (the Dactyliocerate part — the Whitbian) has to be 

 multiplied some eight times and, therefore, the number of contem- 

 poraneous species has to be divided eight times, the diversity of 

 the Dactyliocerates becomes more comprehensible : they represent 

 waves of more or less closely-allied lineages, which develop and 

 tail-off (degenerate) in somewhat similar fashion, pi'oducing 

 homoeomorphous forms. 



It was necessary to be satisfied, in regard to the finely-ribbed 

 Dact3diocerates below the Fish-Bed, that they were not JD. temii- 

 costatiim of the Yorkshire Grey Shales : the}^ are not, but they 

 have a likeness to D. heUanthoides. When really sj^'stematic- 

 research-work on the Dactylioceratidse can be undertaken — at 

 present such work is only in its infancy, — it will doubtless be found 

 that the points of difference between similar-looking forms are 

 more important than is no\v suspected. When these differences 

 can be recorded and brought to bear on species of known date 

 in an extended time-scale, one may expect the Dactylioceratidse to 

 become very useful chronological indicators. 



IX. Summary. 



(1) The bod}^ of the paper deals with certain Jurassic strata 



near Ejqjesmouth on the coast of Dorset; but, as it forms 

 2mrt of a series of preliminary studies in connexion with 

 Jurassic chronology, certain details connected with other 

 localities are noticed. 



(2) A general section of the main mass of Watton Cliff east of 



Eypesmouth is given. 



(3) A detailed section is recorded of a remarkable white litho- 



graphic bed in Watton Cliff, one in the same position as 

 the Upper and Mid-Lias Junction-Bed of the Thorncombe- 

 Beacon area, but differing much in faunal and stratal 

 details. 



(4) This bed shows faunal inversion, presumably due to re- 



deposition of material from older deposits. 



(5) The dating of this Watton Bed is discussed, after pre- 



liminary investigations into the sequence of horizons in 

 the Upper Lias of various areas, in the Junction-Bed and 

 pre-Junction-Bed strata of Thorncombe Beacon. 



(6) A theory of stratal repetition and coalescence is discussed iit 



regard to the Watton Bed. Its main date is taken to b& 

 Yeovilian, Sammatoceras hemera. 



