BRITISH AVONIAN CONODONT FAUNAS 45 
Gnathodus mononodosus Assemblage Zone 
CHARACTERISTIC SPECIES : Gunathodus girtyi simplex Dunn, G. girtyi givtyi Hass, 
G. mononodosus sp. nov., G. bilineatus (Roundy), G. commutatus (Branson & Mehl), 
G. homopunctatus Ziegler, and Spathognathodus cristulus Youngquist. Mestognathus 
neddensis sp. nov. and M. bipluti Higgins are characteristic of this zone in the North 
Crop. 
Limits :_ The lower limit is marked by the first appearance of G. mononodosus sp. 
nov. The upper limit is marked by the first appearance of Gnathodus girtyi collinson 
subsp. nov. The latter subspecies has not yet been recorded from the Midlothian 
section, however. 
REMARKS : This zone is represented by the highest ro ft. of the Dg Subzone and 
by the lowest 6 ft. of the Dg Subzone (Samples CYD 7—3D 7) in the North Crop. 
Our Yoredale samples have yielded no mestognathids. No mestognathids are 
present in the Fife faunas and Collinson & Druce (in press) failed to recover them from 
their Irish Viséan fauna. We agree with their suggestion that the genus has an 
irregular geographic distribution. It is not present in the prolific Upper Mississippi 
Valley faunas of this age. The top of this zone in the Yoredales is the top of the 
Simonstone Limestone (Pq). It is equivalent to the Cu III @ of Germany and to 
the late Valmeyran of the Upper Mississippi Valley. 
Gnathodus girtyi collinsoni Assemblage Zone 
CHARACTERISTIC SPECIES: Gnathodus girtyi collinsoni subsp. nov., G. girtyt 
simplex Dunn, G. girtyi girtyi Hass, G. nodosus Bischoff, G. mononodosus sp. nov., G. 
bilineatus (Roundy). Prioniodina stipans (Rexroad) and P. subaequalis (Higgins) 
are characteristic of the lower part of this zone in the North Crop and in Dunbar. 
Neopriomodus scitulus (Branson & Mehl), N. tulensis (Pander) and Ligonodina levis 
(Branson & Mehl) are characteristic of the higher part in Scotland. 
Limits: The lower limit is marked by the first appearance of G. girtyi collinsont 
subsp. nov. The upper limit is not defined in the present study. 
REMARKS: This zone is represented by the uppermost 18 ft. of the D3 Subzone 
of the North Crop (Samples 3D 8-3D 23). In the Yoredale section it is represented 
by samples Y 29-Y 10 comprising the Middle Pq, the Five Yard, the Three Yard 
and the Underset Limestone (P2:—P2.). | The zone is equivalent to the Cu III y of 
Germany and broadly equivalent to the Lower Chesterian of the Mississippi Valley. 
It may be partly equivalent to the Lower Namurian, depending on where the upper 
boundary is ultimately defined. G. givtyi turritus Collinson & Druce is known to 
extend into the Namurian in Eire (Collinson & Druce in press). It is broadly 
equivalent to the pre-Middle Glen Dean Chesterian faunas of North America. 
The establishment of zonal limits in the upper part of the D succession was 
difficult. The last appearance of Mestognathus beckmanni, Mestognathus bipluti, 
Gnathodus homopunctatus, and the first appearance of Cavusgnathus naviculus, 
Gnathodus nodosus and Gnathodus girtyi turritus were each in turn considered as 
