PLATE XVIII. 
Figs. 1to4. ATACTOPORELLA TYPICALIS, Var. PRAICIPTA, D. VAL. .... sec e eee eee e reece er eeee Sinelclare 223 
land 2. Tangential and vertical sections of a thick expansion, x18. 
3. Surface of well preserved specimen, 18. 
4. A montiferous variety attached to the shell of Orthis subequata Conrad. (See also plate 
XV.) 
Middle third of the Trenton shales, St. Paul. 
HMigey oibO1Ss 1A TACTOPROR ELT AVINS UE DAUM s(SD) sicleistslfaievelelous olelsiclatels'etoteretotelateieleletaielateteters)a)etsleseteieteicievalal=fel=iniele 224 
5and6. Vertical and tangential sections of an example that is typical for the species in all 
respects excepting that the acanthopores are less numerous than usual: X18. (See also 
plate XV.) ' 
7and 8. Tangential section and surface X18, of a variety with larger zocecia than the typi- 
cal and common form of the species. 
Middle third of the Trenton shales, Minneapolis. 
Wigs 29 tO 0 ie HLOMOTRYPH LG AsENS TAR DE IS a Wil pstatstatotelsstotetateteterelejeletaletenetere ciate lelelateiitereletetays eferecsfs\eyalsi=telat =talets 22 
9to1l. Three fragments of the natural size, showing extremes in the size of the branches. 
12. Surface of well preserved example X18, with the mesopores open and distinctly visible 
and the walls thin. 
13.and15. Tangential and vertical sections X18, prepared from the original of fig. 9. The 
walls are thick in the peripheral region and very few cystiphragms are noticeable in 
the tangential section. 
14. Small portion of same tangential section <50 to show the minute structure more clearly. 
16. Central part of a transverse section of same specimen, *18. 
17 and 18. Vertical and tangential sections X18, of a specimen with two series of cystiph— 
ragms, thinner walls, much larger maculx, and more abundant acanthopores than the 
preceding. 
19. Portion of one of the clusters of mesopores 50. 
20. Several zocecia of thesame tangential section 50. 
Middle third of the Trenton shales, St. Paul and Minneapolis. 
Migsa2lkands225 | HOMORRY PEEL AGM Oil LEE O RAW AY SIN-ES [0 islesesedi sta lat= fore lerntele talatelolistarstetel chelated ste aareteratalsvelets(scefer= 280 
21. Tangential section X18, showing, among other peculiarities of the species, the abun- 
dance of the mesopores. 
22. Peripheral part of a vertical section X18, showing the comparatively remote tabulation 
of the tubes. 
Middle third of the Trenton shales, St. Paul. 
Hi GSh123 60) GOs HOMOTR WP HLTA (7) OVEAD AIST s)yerejepetetaerelateteretateyauereicTelatensfaleterstsleletarelelsieietsyetslatelaistele/atal=IsieTe 231 
23 to 26. The basal part of a zoarium and three representative branches of the natural size. 
27. Well preserved fragment of the natural size and a portion of its surface 18. 
28 and 29. Verticaland tangential sections X18. 
30. One zocecium and adjacent parts of the tangential section, x50. 
Galena shales near Cannon Falls, Minn. 
Migs al Loss: LHOMOTRY PEL TA RUSTIO A, | T1s) SD s\eja(slelate/=tal=(orareletsietalsatevelelsleimeteltsialcle rs sialaiateloleteloisisfele Apodnoe 234 
31. Vertical section <18. 
32. Two portions of a tangential section X18, the upper part giving the appearance imme- 
diately beneath the surface while the lower represents a deeper level. 
33. Small portion of the mature region of the tangential section x50. 
Upper part of the Hudson River group near Spring Valley, Minn. 
