\)(t The American Geologist. February, looi. 
(Minnesota), is a much larger, stronger and thicker shell, and 
the beds at St. Croix, where it is so numerous, are considered 
later or Upper Cambrian, indeed, might be so considered upon 
this evidence alone. 
Oholella is a primordial genus, but the evidence of in- 
creased size, with geological age is not at all clear. The N. Y. 
species, O. crassa, is much larger than O. polita of Wisconsin, 
but if Lingula prima Conrad, is Hall's O. polita, as Prof. 
Whitfield claims, the argument still holds as the western form 
in this exact instance is much larger than its eastern and ear- 
lier representative. 
Turning to Discina and Crania we find this general propo- 
sition still obviously sustained. If under Discina, so far as the 
elements of this inquiry go, we group the discinoid shells 
Discinisca, Orhiculoidea, Schizotreta, Lindstroemella, Roemer- 
ella, we find the increase of size well shown. The small Or- 
bicitloideas of the L. Silurian becomes the large O randalli and 
grandis of the Devonian. 
Lingulas increase in their dimensions from their first ap- 
pearance, and it is the acuminate type which prevailed in the 
earliest faunas (Clarke) which is succeeded by the broader, 
more developed forms, while, as indicated by Clarke, Barroi- 
sella, with its deltidial hardenings, coming in in the Devonian, 
continues the Lingula type until there is "an evident tendency 
to span the interval between the so-called inarticulates and ar- 
ticulates." 
Trematis, as confined apparently to American Silurian 
faunas, shows a sensible increase in size and general develop- 
ment, if terminalis of the Trenton is compared with niille- 
punctata of the Hudson River group. 
In the articulate brachiopods this same organic impulse is 
distinctly presented. In the feature of increasing articulation 
Dr. Clarke has said, "with rare exception these modifications 
in each group appear to be progressive, extending along cer- 
tain lines of development, and finally acquiring an extravagant 
manifestation which may terminate abruptly or result in the 
degeneration and obsolescence of some of the parts." 
The spiriferoids reach a climax of development in the De- 
vonian and Carboniferous. The increased size and the ampli- 
fied cardinal and hinge features reaching such complexity as 
