SUPPLEMENT.— CHRONOLOGY. v 



but a small portion of the side of the whorl is shown, it may be called sub- 

 concav limb ilic ate ; when" more of the preceding whorl is exhibited, the conch would 

 be gradumbilicate. 



Graterumbilicate, having a deep basin-shaped umbilicus like that of Ammonites 

 Blagdeni. 



Latumbilicate, with a wide umbilicus. 



Angustumbilicate, with a narrow umbilicus. 



Goncentrumbilicate, having an umbilicus in which the whorls coil regularly 

 around the central axis at a gradually increasing distance. 



Excentrumbilicate, having an umbilicus in Avhich the whorls do not coil regu- 

 larly around a central axis. The umbilicus shows a more or less sudden expan- 

 sion after a certain period. 



Particular attention should be given to whether a species is excentri- or 

 concentri-latumbilicate. Frequently a species may be angustumbilicate in youth, 

 but may become excentrilatumbilicate in the adult state. 



The Septa. — The following terms in connection therewith are required for the 

 sake of brevity in definition. 



Bensiseptate, a specimen with septa close together. 



Paucisepttate, when the septa are distant. 



Ornatilobate, when the septum is considerably branched, so as to form a 

 complex suture-line. 



Inornatilobate, when the septum is not much branched. 



By the use of these technical terms the descriptions will be rendered much 

 more concise, and the comparison of species will be facilitated. 



Chronology, — One other matter yet remains, and that is how to indicate the 

 sequential occurrence of the different species. In this Supplement the chrono- 

 logical system will be adopted ; and each species will be dated, as regards the time 

 of its existence, by means of the chronological unit, the term hemera} 



For the purposes of this Monograph the term " Inferior Oolite" has been con- 

 sidered to embrace the deposits from the base of the Cotteswold Sands of Frocester 

 Hill to the top of the limestone beds of Broad Windsor, inclusive. The time 

 which it took to deposit these and all their intervening strata is now divided into 

 twenty-three hemeras, whereby it is possible to express the date and sequence of 

 species with considerable and very necessary exactitude. The list of these 

 hemera?, named after their principal Ammonites, is as follows : 



1 " The Bajocian of the Sherborne District," ' Quart. Jouru. Geol. Soc.,' vol. xlix, p. 481. 



