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 32. 



Variety MINOR, n. 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 zooscial 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 zooecial apertures, and the thicker 

 walls or inter-apertural spaces, distinguish the species without much trouble from 

 associated species of Homotrypa and Callopora. 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, n. sp., 



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 



