10 BRITISH AVONIAN CONODONT FAUNAS 
Arkansas (1956A), the Maury Shale of Tennessee (1956), the Barnett Formation 
(1953) and the Chappel Limestone of Texas (1959). 
Scott & Collinson (1961) described a fauna from the Louisiana Limestone and 
from the McCraney Limestone. 
Youngquist & Patterson (1949) described conodonts from the Prospect Hill Sand- 
stone of lowa. The fauna of the Lower Mississippian Wassonville Dolomite of Iowa 
was described by Youngquist & Downs (1951). Youngquist, Miller & Downs (1950) 
described Burlington conodonts from Iowa. 
Rexroad (1957) described Chester conodonts from I[linois and later (1958) from the 
Glen Dean Formation. Rexroad & Clarke (1960) described Glen Dean conodonts 
from Kentucky, Virginia and West Virginia. Golconda Group conodonts were 
described by Rexroad & Jarrell (1961). Those of the Kinkaid Formation in Illinois 
were described by Rexroad & Burton (1961) and those of the Paoli and equivalent 
formations in Illinois by Rexroad & Liebe (1962). 
From the work of these and other workers, it became clear that conodonts were 
abundant in Mississippian rocks and were useful for correlation of strata. As a 
result, the Illinois State Geological Survey, in co-operation with the University of 
Texas, the State University of Iowa, Texas Technological College, the University of 
Houston, and the Indiana Geological Survey, conducted a programme of research in 
the Mississippi Valley. In 1962 Collinson, Scott & Rexroad published a paper in 
which they described 17 conodont biostratigraphic zones, which were present in the 
Mississippian rocks of the Mississippi Valley. The limits and characteristic species 
of each zone were described and they also attempted to correlate these zones with the 
conodont zones present in Germany. 
Subsequent workers have systematically described the faunas of the biostrati- 
graphic zones established in 1962. Thus Rexroad & Collinson (1963) not only 
described the conodonts of the St. Louis Formation, but also indicated, described 
and illustrated the species characteristic of the Taphrognathus varians—A patognathus 
Assemblage Zone and of the A patognathus ? geminus—Cavusgnathus Assemblage Zone. 
In the same way Rexroad & Scott (1964) when describing the conodont faunas of 
the Rockford Limestone and the lower part of the New Providence Shale of Indiana 
described and illustrated the conodont fauna characteristic of the Szphonodella 
tsosticha—S. coopert, Gnathodus seniglaber—Pseudopolygnathus multistriatus, Bactro- 
gnathus—Polygnathus communis and Bactrognathus—I aphrognathus Assemblage Zones. 
They also showed in tables the numerical distribution and stratigraphic ranges of 
specimens. 
Rexroad & Collinson (1965) provided the same data for the Taphrognathus varians— 
Apatognathus Assemblage Zone, when describing the conodonts of the Keokuk, 
Warsaw and Salem Formations of [linois. 
Rexroad & Furnish (1964) referred their fauna from the Pella Formation of South- 
Central Iowa to the Gnathodus bilineatus—Cavusgnathus charactus Assemblage Zone of 
the Mississippi Valley, and to the St. Genevieve Limestone in particular. 
Rexroad & Nicoll (1965) described the faunas of the Menard Formation, which 
they referred to the Kladognathus—Cavusgnathus naviculus Zone, drawing the lower 
