BRITISH AVONIAN CONODONT FAUNAS 41 
disappearance of S. tvidentatus (E. R. Branson), Clydagnathus darensis gen. et sp. 
nov., C. unicornis gen. et sp. nov. and S. crassidentatus (Branson & Mehl). 
In the Avon Gorge there are two subzones : a lower subzone characterized by the 
presence of S. costatus costatus (E. R. Branson), S. costatus sulciferus (Branson & 
Mehl) and S. cf. cvistulus Youngquist & Miller, with very few pseudopolygnathids 
present, (Samples K 18-Z g) and an upper zone of pseudopolygnathids (Samples 
Z 10-Z 25). In the subzone of abundant pseudopolygnathids, Pseudopolygnathus 
primus (Branson & Mehl) and Pseudopolygnathus cf. dentilineatus are abundant. 
Pseudopolygnathus multistriatus Mehl & Thomas has its lowest stratigraphic occur- 
rence in Sample Z 23. 
Polygnathus lacinatus Assemblage Zone 
CHARACTERISTIC SPECIES : Polygnathus lacinatus s. s. Huddle, Polygnathus 
lacinatus asymmetricus subsp. nov., Pseudopolygnathus nodomarginatus (E. R. 
Branson), Spathognathodus pulcher Branson & Mehl and Pseudopolygnathus multi- 
striatus Mehl & Thomas. Gunathodus antetexanus Rexroad & Scott, is present near 
the base of the zone in the North Crop and at Farlow. 
Limits: The lower limit is marked by the incoming of the P. Jacinatus group and 
of Gnathodus semiglaber (Bischoff). It is also a zone of abundant Pseudopolygnathus 
multistriatus Mehl & Thomas. The upper limit is marked by the incoming of 
Pseudopolygnathus cf. longiposticus (Branson & Mehl). 
REMARKS: This zone occupies the uppermost part of the Z sequence in the North 
Crop and occurs near, but not at the top of, the Z Zone in the Avon Gorge, (North 
Crop Samples ZLA 29-ZL 19 : Avon Gorge Samples Z 26-Z 32). It is correlated 
with the unconformity at the base of the Sedalia Formation of North America, with 
the lowest Cu II @-y of Germany and with the base of Tng, of Belgium. The 
presence of G. semiglaber and P. multistriatus in this zone indicates its broad equiv- 
alence to the Sedalia Formation, but the fact that it also contains abundant G. 
delicatus also implies a similarity to the underlying Upper Chouteau Formation. 
These two formations are separated by an unconformity in the Mississippi Valley, 
below which abundant G. delicatus occur, but above which this species is absent. It 
seems probable that beds Z 26-28, in which the two groups of species overlap, are 
broadly equivalent to the Chouteau—Sedalia unconformity time interval of the 
Mississippi Valley (see also p. 59). 
Polygnathus lacinatus—Pseudopolygnathus cf. longiposticus 
Assemblage Zone 
CHARACTERISTIC SPECIES : G. semiglaber (Bischoff), G. delicatus Branson & Mehl, 
P. cf. longiposticus (Branson & Mehl), Polygnathus lacinatus s.s. Huddle, Spatho- 
gnathodus pulcher Branson & Mehl, Guathodus simplicatus sp. nov. and Gnathodus 
avonensis Sp. Nov. 
Limits: The lower limit is marked by the lower limit of abundant P. cf. longi- 
posticus (Branson & Mehl). It is also a zone of abundant Guathodus. The upper 
