A CIRCUM-INSULAR PALZOZOIC FAUNA. gII 
The exceptions to this rule are: Productella, Goniatites and 
Microcyclus. The fact of the recorded distribution of Productella 
points either to a dual origin of the genus, or to an imperfection 
in the record, or, possibly to an exceptional migration across or 
around the barrier in spite of the general effectiveness. The 
name Gontatites is applied to a heterogeneous assemblage of 
forms, which have been divided into numerous genera by Hyatt 
and other recent writers, and if the Chouteau species were referred 
to these more limited divisions, it is probable that they would 
not appear as exceptions. Mucrocyclus occurs in the Devonian 
fauna at Devil’s Bake Oven in southern Illinois, where the east- 
ern faunas approach nearest geographically to those of the west. 
It is possible that this fauna lived during the period of sub- 
mergence of the land barrier, and that AZtcrocyclus in this locality 
is really a representative of the western and not of the eastern 
fauna. | 
Taking the generic evidence as a whole, it points to the dual 
origin of the fauna. 
Specific evidence of the origin of the littoral fauna about the Ozark 
Island.—In the consideration of the specific evidence of the 
origin of the Chouteau fauna each species has been studied with 
special reference to its evolution from preéxisting species. As 
might be expected, the evolution of the species can be traced 
in only a few instances comparatively, but this evidence also, so 
far as it is available, points to the dual origin of the fauna. Each 
of the species whose evolution can be traced will be briefly con- 
sidered. . 
Athyris hannibalenis, Swallow. ‘This species is not uncommon 
in the yellow Chouteau Limestone. It most closely resembles 
_ A. lamellosa, L’Eveille, of the succeeding beds. It is generally 
less transverse than that species, though this character is varia- 
ble, and individual specimens may be found which are quite equal 
to A. lamellosa in this respect. The concentric lamella on the 
exterior of the shell are somewhat closer together than in 
A. lamellosa from the Burlington Limestone. It is intermediate 
in its characters between A. sfiriferordes, Eaton, of the New 
