PLATE XXV. 
Mies FO Mis) SAT OST OMAGH UE: ROTA HOU) litte iaeieteveverterareraisy slate aratstetofaceten steleloretateferetacenerel nolo siete eae eke eters 
land 2. A small and a large specimen. 
3. Surface of the larger specimen, which is a typical example of the species, *9. 
4. Tangential section, prepared from the same specimen, *18. 
5 and 6. Surface of the var. circulare x9 and X18. 
7. Tangential section of a speciinen with very thin walls and exceedingly small acantho- 
pores, X18, The shaded spaces are mesopores. 
8and9. Tangential sections of two specimens of the var. circulare, X18, the first with very 
thick wall, the second with them thinner and in this respect agreeing with figs. 5and 6. 
10. Portion of the axial region of a transverse section of a typical specimen, X18. 
11. Vertical section of the natural size and a portion of same *18; typical. 
Lower third of the Trenton shales, Minneapolis. 
Re Saal 2 show bse BATOSTONGA (MEA GIN OPORAG MIB D stetsrerecatasustelsumsnsttee taeyareieiat el i-tete ven atstete (oteteisietel steers chee picts 
12, A rather small specimen of the natural size. 
13. Tangential section <18. 
14, Part of the axial region of a transverse section X18. 
15. Vertical section x9. 
Middle third of the Trenton shales, Minneapolis and St. Paul. 
IMERR OO Pa eey too ON NAOT aS) dacdoomaedat ookonteudrocotne Qao0 .bsaaushototod med adGodounGG 
16 to 22. Seven figures illustrating the extreme variability displayed by tangential sections 
of this species; x18. Figs. 16 and 17 represent different parts of the same section, 18 
shows the tubes just after bending out of the axial region, 16, 19 and 20 are of common 
occurrence, and 21 and 22 show extremes rarely met with. 
23 and 24. Halves of two vertical sections *9, the first normal, the second indicating some 
disturbance in the development of the tubes. 
25. One zocecial tube of a vertical section of the basal expansion 18. 
Middle third of the Trenton shales at St. Paul and Minneapolis. 
Migs 20500 28.0 DATOSROMAUMON DU OSUNE Msgs Dita cletsteisrecselste tricia ieleleectelietasitclevs ie steeietensiieie erator 
26. An average specimen of the natural size. 
27 and 2&. Surface of a specimen with numerous closed mesopores *9 and a small portion of 
same X18. In many specimens the interspaces are narrower than is shown in these 
figures. 
Upper third of the Trenton shales near Cannon Falls, Minnesota. 
Wigs e290 SO SA LOSTONAS HUT TMELT EY Ello) SI) cia tslet a Wetoncteied slate alaralskave(-intatore aie’ sfalekole/eloleietererstcialemeier(eiateyetalat ates 
29 to 31. Three fragments of the natural size. 
32. Small portion of a tangential section 18 of a specimen having asurface like fig. 34. 
33. Tangential section of a less matured example X18. 
34. Surface of an average fully matured example *9. 
35, Surface of a young example (corresponds to the lower part of fig. 33) *18. 
36. Surface of a fragment regarded as illustating a condition of extreme age, 18. 
Galena shales near Cannon Falls, Minn. 
293 
