358 The American Geologist. J""^- ^^o^. 
The shallow waters surrounding this area of long contin- 
ued, progressive subsidence contained a wealth of animal life. 
This life, owing to geologic causes not yet definitely worked 
out, was subject to migrations. The character and direction 
of these migrations is certain to prove a subject of investiga- 
tion m the near tuiure. 
As a result of these migrations, some species entered the 
field here under investigation, and, after a brief existence, dis- 
appeared again. 
For instance, at some time previous to the Cincinnati epoch, 
the genus Strophomena^ disappeared from southern Ohio, In- 
diana, and Kentucky. In the lower Utica, Strophomena hal- 
liana is a rare fossil. At the close of the deposition of the 
Mount Hope beds, Strophomena planoconvexa entered the- 
field, and, after forming part of a layer a few inches thick in 
the vicinity of Cincinnati, it disappeared again. A little later, 
during the deposition of the Fairmount beds, Strophomena sin- 
iiata had a nearly equally brief existence. No other Strophom- 
enas are known until the base of the Richmond stage is reached. 
Strophomena planoconvexa and Strophoinena sinuata belong 
to different sections of the genus. Nevertheless their occur- 
rence within fifty feet of one another separated by considerable 
intervals from other horizons containing Strophomena may 
have some significance, since this suggests the possibility of 
brief connections of the sea near Cincinnati during the earlier 
Lorraine with some other area in which the development of 
the genus Strophomena was more continuous. It would be 
interesting to know from what source the species migrated to- 
wards Cincinnati. The forms identified as Strophomena 
planoconvexa near Maysville, Kentucky, have a vertical range 
of fifteen feet, being very abundant in the lower six feet, while 
the typical forms at Cincinnati have a range of only a few 
inches. During the deposition of the Richmond, connection 
with the region from which Strophomena sinuata and Str. pla- 
noconvexa migrated may have been established again, the first 
species being represented in the Richmond by Strophomena 
sulcata, while the latter is represented by an undescribed 
species differing only in its greater width and shorter length. 
t "The Lower Silurian Deoosits of the Upper Mississippi province. WiN- 
CHELL and Ulrich,1897, Geo'l.Minn., vol.iii, part 2, pp. ciii, civ, also 384-396. 
