PLATE XXIV. 
igeas bo.) EeMrP HR A GNAY OTTAWA NSE: (MOOLG)) so cieiaictere see ereletsieters els ereieveloeiossioraieartteteierare ti cieelsinicierrcitteraete 300 
land2. Two portions of a tangential section x18, showing differences due to age. 
3. Vertical section <9. ; 
4. Small portion of the peripheral region of the same X35. 
Upper part of the Galena shales, Mantorville, Minn. Survey Museum, Reg. No. 6002. 
Migss5)/ GOOF Emme rR A\GMCAS Ti VA'S TMi LUDIIT erareretousre rate leteieretebate eretetsvel cieietoreiorevevietelor ioral ternicraleiete raters ieteretetetetereiets 299 
5. Vertical section <9. 
6,7,9and14. Four tangential sections <18, exhibiting individual variations. 
8. Small part of a transverse section X18. 
10 to 13. The surfaces of four specimens *9, fig. 10 representing a ood average, fig. 12 the 
appearance of young examples, and figs. 11 and 13 the fully matured conditions. 
15 to 19. A group of five specimens, three of them of the average size, while 17 is the largest 
and 18 one of the smallest seen. 
Lower third of the Trenton shales at Minneapolis, and the Galena shales near Cannon 
Falls, Minn. 
Bigs; 20it0i 23. EEE MP ERA GMAW EN OEM UR AUD IL SDD erate etal cfs Sieh eintelcealele) -lecetctels eteietelie ede fekstonsdetele eistetet eters telets 301 
20. Vertical section X18. 
21 and 22. Tangential sections X18, the first showing the fully matured condition, the 
second a younger stage in the development of the zoarium. 
23. Surface of a well preserved example showing the incomplete diaphragms in the zoccial 
cavities, <9. 
Galena shales near Cannon Falls. 
Migs 24 FOr le STROMAL OLR VAC OiveATUAG, Mey Ds etetererarsistocerelol cterctarsiet stat evoloietaveisrebetelatcteleterelejaucietolerensterete etateietere 302 
24. Surface of an old example 9. 
25, Well preserved specimen consisting of several layers, natural size. 
26. Small portion of one of the first layers of same X18, showing an aged condition. 
27. Part of the last layer of same <18, showing appearance of surface in the youngest stage 
observed. ‘ ; 
28 and 29. Tangential and vertical sections <18, of an example consisting of three distinct 
layers of zocecia 
30 and 31. Another pair of sections, vertical and tangential, the latter illustrating a more 
aged condition than is shown in fig. 28; X18. 
Middle third of the Trenton shales, St. Paul and Minneapolis. 
