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AUG. F. FOERSTE 
eluding also the Conularia heymani described in this article from 
the Plattin limestone of Missouri. The little trilobite Endym- 
ionia heUatula Savage, described from southern Illinois and north- 
eastern Missouri, apparently finds its nearest relative in the Cana- 
dian Endymionia meeki Billings of Quebec and Newfoundland. 
In a similar manner the Orchard Creek shale at the base of the 
Alexandrian (Medinan) of southern Illinois and adjacent Mis- 
souri contains such Ordovician elements as Cyclocystoides, Bys- 
sonychia, Lyrodesma, Phragmolites, Isotelus, and Ceratopsis, The 
Calymene duhia of Savage belongs to the same group as Calymene 
christyi and Calymene platycephala, from the Richmond and Tren- 
ton respectively, for which the writer recently proposed the 
generic term Platycoryphe. 
The association of such Ordovician types with others of Silu- 
rian character suggests the proximity of southern Illinois and 
southeastern Missouri, in early Silurian times, to areas in which 
Ordovician types still thrived. The associated Silurian forms 
evidently must have originated as some more distant source, and 
must have entered the eastern Missouri areas only as migrants. 
DESCRIPTION OF SPECIES 
1. Tetradium fibratum Salford. 
Most species of Tetradium of Black River age consist of more 
or less dissociated groups of cells, while massive growths pre- 
dominate in later strata. 
At the top of the Plattin limestone, in Ralls County, a species 
with more or less flattened massive growth occurs, but in com- 
paratively few numbers. One specimen, found at Conn’s Ford, 
consists of a circular corallum, 32 cm. in diameter, and 6 to 8 
cm. thick. Along the exposed surface the corallites are arranged 
in straight or curved rows which intersect each other at angles 
varying from 80 to 90 degrees. Usually about 7 corallites occur 
in a width of 5 mm., varying locally from 6 to 8 in the same dis- 
tance. The corallites are more or less quadrate in cross-section, 
single septa extending inward from the middle of each wall, and 
almost or fully reaching the center of the corallites. Similar 
specimens occur at the J. H. Smith and Yeager localities. 
