266 THE PALEONTOLOGY OF MINNESOTA. 
{Eridotrypa mutabilis 
all through the axial region at intervals, averaging about twice the diameter of a 
tube. As the tubes are about to open at the surface the diaphragms increase in 
number, and immediately thereafter the walls are greatly thickened, and mesopores 
developed. The latter were unusually numerous in the sections drawn in figs. 
31 and 382. ; 
Variety MINOR, ”. var. 
PLATE XXVI, FIGS. 20, 21, 29, 30. 
This name may be attached to the small form represented by the figures cited. 
The surface magnified is generally very much as shown in fig. 21, the zocecial walls 
being thinner than in typical mutabilis. But the principal peculiarities are to be 
found in the axial region, as shown in vertical sections. First, the central tubes are 
unusually large and their walls more wavy than in typical mutabilis; second, the 
tubes altogether seem to have been developed more regularly, and their width in the 
peripheral region somewhat less; and third, diaphragms are wanting throughout 
the greater part of the axial region. Under ordinary circumstances these differ- 
ences would be considered as of specific value, but in this instance, knowing the 
extreme variability of the species, I cannot credit them with more than subordinate 
importance. 
The smaller size of the branches, oblique zoccial apertures, and the thicker 
walls or inter-apertural spaces, distinguish the species without much trouble from 
associated species of Homotrypa and Cajlopora. Despite its variability, I have always 
' found it one of the easiest of the numerous Trenton species to identify off-hand. 
_ Formation and locality.—Very common in the Galena shales at many localities in Goodhue, Dakota 
and Ramsey counties in Minnesota; also at Decorah, Iowa, and in the Galena at Neenah and Oshkosh, 
Wisconsin; in the shaly portion of the Trenton group at many points in central Kentucky; also at Nash- 
ville, Tennessee, and Ottawa, Canada. Specimens referred to the var. minor are to be found also in the 
upper third of the Trenton shales at St. Paul. 
Mus. Reg. Nos. 5541, 6009, 7561, 7603, 7623-3, 8034, 8050, 8079. 
ERIDOTRYPA EXIGUA, . SDp., 
PLATE XXVI, FIGS. 17-18. 
Zoarium small, branches very slender, several hundred fragments varying in 
diameter from 0.6 to 1.0 mm.® bifurcations apparently remote. Some of the frag- 
ments are pointed at the lower end, indicating a free condition of the zoarium, or an 
articulation like that of Escharopora, The eastern form of the species is usually a 
little stronger than the average of the Minnesota types, the specimens seen from 
