28 The American Geologist. July, 1896 
Regarding the lower limit there has been until very recently 
no satisfactory horizon decided upon. As the so-called Per- 
mian in this part of the continent is essentially co-extensive 
with the Oklalioman series the discussion of the former may 
be properly considered as applying to the latter. This has 
already been referred to in connection with the location of 
the summit of the Missourian. The Oklahoman therefore 
reached downward to the top of the Cottonwood limestone, 
which sharply separates it from series beneath. 
NOTES ON CAMBRIAN FAUNAS THE GENUS 
MICRODISCUS. 
By G. F. Matthew, St. Jolui, N. H., Canada. 
This genus, which by the small number of its pleural joints 
is almost on a par with Agnostus. exhibits a variation of form 
according to the age of the beds in which it is found, showing 
a development of the type comparable to that in Agnostus, 
but on ditt'erent lines. 
In the Olenellus fauna three types of the genus Microdiscus 
are found, distinguishable by the form and length of the gla- 
bella and the number of rings in the axis of the pygidium. 
Two of these types show in the Paradoxides zone, but are re- 
markable for the great development of the occipital spine. 
This spine is not merely an extension of the crest of the occip- 
ital ring, but carries with it the posterior part of the glabella ; 
hence like Agnosais these species have tiie occipital ring, ei- 
ther quite obscure, or entirely effaced. Two types were long ago 
recognized in the Paradoxides beds by the late professor C. 
F. Hartt, who gave to one the name Dawsonia* and to the other 
Eodiscus.f 
We cannot trace any increase of tlie number of joints in the 
thorax that may seem relative to the geological age of the 
form, except that while speriosKs of the Olenellus fauna has 
three joints, puncf((fif.s of the Paradoxides fauna has four. 
On the other hand, however, sculptiis and tlawsoni, both of the 
lower Paradoxides fauna, have respectively four and two]; 
joints. 
*Acadian Geology, 2d ed. p. 055, fig. 2*28. 
tU. S. Gtiological Survey, Bulletin 10, p. 24, pi. ii, figs, la to c. 
jIn M. dairsoni this is for an individiuil not fully grown. 
