334 THE PALEONTOLOGY OF MINNESOTA. 
[Distribution 
picking to secure of a species a complete series of specimens from less than 1 mm. 
in length to the adult size. Such series are of great value in classification, and 
much yet remains to be done in this direction.* A great deal can also be learned 
by local collectors, in regard to the evolution, introduction and disappearance of 
the species in these shales. 
Near the top of the Trenton shales, new forms are introduced gradually, so that 
in the overlying Galena deposits the brachiopod fauna is changed. With the intro- 
duction of shales containing Clitambonites diversa, the marking species of this horizon, 
the fauna is noticed to disagree more or less in specific expression with that of the 
underlying Trenton shales. A number of forms are common to both horizons, but 
there is a perceptible difference in them. Ascending towards the middle Galena, the 
older species drop out, and new ones take their places more and more rapidly. In con- 
nection with this faunal change, there is also a lithological one. The Trenton shales 
are greenish in color, but change easily to a yellow with a coarser texture, before the 
Clitambonites horizon is introduced. The strata then become more and more sandy 
in the northern exposures of the Galena, which towards the south is altered into a 
compact, thin-layered limestone series. 
Near the middle of the Galena there is another brachiopod horizon quite dis- 
tinct from any below it. At some localities the species found here are preserved 
as casts while in others the shell remains. The characteristic species of this horizon 
are Orthis meedsi var. germana, Rajinesquina deltoidea, Strophomena trilobata, Pledam- 
bonites gibbosa, Zijgospira uphami, Cyclospira bisulcata, Schizotreta pelopea, and Lingu- 
lasma galenensis. From these beds to the Hudson River group above, the fauna is 
rather meager, and little collecting has been accomplished. 
In the upper member of the Hudson River deposits, the brachiopods are again 
numerous in individuals and species. Its fauna agrees with that of the upper por- 
tion of the Cincinnati group of the Ohio valley. The fossils are preserved in a semi- 
siliceous condition. Numerous outcrops of this formation occur in the southern 
portion of the state, but the fauna appears to be localized, and not well preserved 
nor abundant except in the region of Spring Valley. 
Below the Trenton limestone, but one brachiopod (Lingula morsii) is known in 
the St. Peter sandstone; none in the Shakopee formation; but several, as yet 
unstudied, in the Lower Magnesian. In the St. Croix formation, however, 
brachiopods are abundant but mainly of inarticulate species. 
* See " The Development of some Silurian Brachiopoda," 1)V Beecher and Clarke : Mem. N. Y. State Mus.. vol. i. no. 1, 
1889 : " Development of the Brachiopoda." pt. i, by C. E. Beecher, Am. .lour. Sci.. vol. xli, IS'Jl : and " Development of Bilob- 
ites," by C. E. Beecher, Am. .Jour. Sci.. vol. .\li, 1891. 
