34 BRITISH AVONIAN CONODONT FAUNAS 
of Pseudopolygnathus in the Lower Z Zone of the Avon Gorge, the genus is absent in 
strata of similar age from the North Crop. In these latter faunas it is presumably 
represented by functional homoeomorphs in the natural conodont assemblages. 
These could well be spathognathodids, from which Pseudopolygnathus seems to have 
arisen. The Lower Z Zone assemblage is replaced by abundant P. multistriatus Mehl 
& Thomas near the base of the Zz Subzone, which is in turn replaced by abundant 
P. longiposticus (Branson & Mehl) towards the top of the Zz Subzone. The genus is 
rare in the Lower C; Subzone. 
The genus Gnathodus is one of the longest ranging in the Avonian, but its restricted 
and distinctive species have been particularly useful in some of our correlations. Its 
oldest occurrence is near the base of the Zz Subzone, where G. delicatus Branson & 
Mehl is found. On the North Crop, G. sitmplicatus sp. nov. also occurs in Lower Z 
Zone faunas. G. delicatus is associated near the top of the Zz Subzone with G. semi- 
glaber (Bischoff), a distinctive Upper Z Zone species, which is in turn replaced by G. 
antetexanus Rexroad & Scott at the top of the Z Zone. G. punctatus (Cooper), G. 
avonensis sp. nov. and G. simplicatus sp. nov. also occur in the Upper Z Zone. A 
single specimen of G. cuneiformis Mehl & Thomas was collected in the middle of the 
S; Subzone. Gnathodus is rare or absent in the higher C and S faunas, but is 
represented by several distinctive species in the D Zone. G. bilineatus (Roundy) 
appears at the base of the Dg Subzone, together with G. girtyi girtyi Hass. In the 
highest part of the Dz Subzone G. girtyi simplex Dunn, G. commutatus (Branson & 
Mehl), G. mononodosus sp. nov. and G. homopunctatus Ziegler appear. G. girtyi 
collinsoni sub. sp. nov. appears in the middle of the Dg Subzone, and G. girtyi 
turritus Collinson & Druce near the top (Fig. 11). 
The genus Spathognathodus has an extended range, being present throughout the 
Avonian. The dominant species of the Lower K Zone is S. plumulus sp. nov., which 
is represented by several subspecies. The Upper K Zone is marked by a variety of 
spathognathodids, including S. elongatus (Branson & Mehl), Spathognathodus cf. 
vobustus (Branson & Mehl), S. anteposicornis Scott, and S. tridentatus (E. R. Branson). 
The three former species, though never abundant, extend into the Lower Z Zone, 
while the latter, together with S. cvassidentatus, which first appears in the basal K 
Zone, is abundant in the Lower and Middle Z Zone. S. costatus costatus (E. R. 
Branson) and S. costatus sulciferus (Branson & Mehl) appear in the Lower Z Zone in 
the North Crop and are abundant in the middle part of the Zone. S.cyrius (Cooper), 
a rare species throughout the K and Lower Z Zones, overlaps the lower occurrence of 
these species, and S. cf. cristulus Youngquist & Miller is also present, extending 
upwards into the C Zone. S. pulcher Branson & Mehl occurs high in the Z Zone and 
S. coronus sp. nov. in the C Zone. S. scitulus (Hinde) is present in the Canima 
Oolite and extends into the D Zone. S. cristulus Youngquist & Miller is character- 
istic of the Dz Subzone and S. campbelli Rexroad of the Ds. 
Polygnathus first appears near the base of the K Zone and extends to the top of C). 
Polygnathus communis Branson & Mehl extends from near the base of the K Zone to 
the middle of the Laminosa Dolomite. Polygnathus inornatus inornatus Branson & 
Mehl, P. lobatus lobatus Branson & Mehl, P. inornatus rostratus subsp. nov., P. 
al 
