Notices of Memoirs — S. 8, Buchnan — Ammonite Zones. 503 



tlie Sonninice of tlie Covcnvum-zoxiQ are, biologically, of an earlier 

 type than those of the Soioerbyi-zone. 



Continental authors find a marked stratigraphical and palaeonto- 

 logical break between the MurcMsonce- and Sowerbyi-zones ; and 

 they wish to draw, at this point, a dividing line between Lias and 

 Oolite, or between Toarcian and Bajocian. It is suggested that the 

 absence of the Goncavum-zone is the cause of this break ; and, in 

 former papers to the Geological Society, the author, in supporting 

 the Continental plan, regarded the Concavum-zone as Toarcian. 



In the Bradford Abbas district there is a break above the 

 Concavum-zone. So far as is known at present, Dundry is the only 

 locality showing a complete sequence ; but some years ago a 

 quarry — Coombe, near Sherborne — was open, and it yielded a large 

 series of Ammonites indicating a fauna agreeing with the Sowerbyi- 

 zone as illustrated by Continental authors. This quarry has been 

 closed for years ; and nothing is known as to how the strata are 

 situated with regard to the Concavum-zone below, or with superior 

 horizons. It is richer than Dundry, and is, practically speaking, 

 unique among Inferior-Oolite exposures. It is the only locality in 

 England which yields this particular fauna. So far as is known, 

 the true Sowerbyi-zone is absent from all quarries in Dorset and 

 Somerset, with the exception of Coombe and Dundry ; and, therefore, 

 the majority of exposures in the district fully support the Continental 

 geologists in their contention as regards a dividing line, 



Waagen places a zone of Am. Sauzei above the Sowerhyi-zone ; 

 and an horizon with this species and with a particular fauna is shown 

 in ' the marl with green grains ' at Frogden quarry, near Sherborne. 



Above this is the zone of Am. Humph-iesianus, in which the fauna 

 Stephanoceras and Sphceroceras predominate. This is the equivalent 

 of the Coronaten-schichten of Quenstedt's Brauner Jura S. The upper 

 part of the Brauner Jura d is the Bifurcaten-schichten ; and this 

 corresponds with the Cadomensis-heds of Frogden — an horizon which, 

 containing a fauna distinct from the Humphriesianus-zone, may 

 therefore be known as the Cadomensis-zone. 



The strata above this horizon have usually been called the ' Parhin- 

 soni-zone.' There are several objections to this name; and the strata 

 are capable of more subdivision. 



The bed at Halfway House which yields the large ParTcinsonice is 

 superior to the Cadomensis-zone. It may be called the Truellii-zone. 

 At the top of the limestone of the Broad- Windsor district Stephan. 

 zigzag and species of Morphoceras are found ; and this is a still 

 higher horizon [Zigzag-zone). Just below the Fullers' Earth of 

 this same district, in the Fullers' Eai'th itself of Eype, but in the 

 upper white limestones (about 25 feet thick) of the Bradford-Abbas 

 district, are found Oppelia fusca and other species indicating a still 

 higher horizon. It is suggested that the white limestone of the 

 Bradford -Abbas district is contemporaneous with the so-called 

 'Fullers' Earth clay' of Eype. This horizon may be called the zone 

 of Oppelia fusca ; and whether this zone belongs to the Inferior 

 Oolite or to the Fullers' Earth depends on whether the observer be 



