PLATE XVIII. 



PAGE. 



Figs. 1 to 4. ATACTOPORELLA TYPICALIS, var. PR^CIPTA, n. var 223 



1 and 2. Tangential and vertical sections of a thick expansion, x!8. 



3. Surface of well preserved specimen, xig. 



4. A montiferous variety attached to the shell of Orthis subcequata Conrad. (See also plate 



XV.) 

 Middle third of the Trenton shales, St. Paul. 



Figs. 5 to 8. ATACTOPORELLA INSUETA, n. sp 224 



5 and 6. 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: xi8. (See also 

 plate XV.) 



7 and 8. Tangential section and surface X18, of a variety with larger zooecia than the typi- 

 cal and common form of the species. 

 Middle third of the Trenton shales, Minneapolis. 



Figs. 9 to 20. HOMOTRYPELLA INBTABILIS Ulr 22 



9 to 11. Three fragments of the natural size, showing extremes in the size of the branches. 



12. Surface of well preserved example xis, with the mesopores open and distinctly visible 

 and the walls thin. 



13 and 15. Tangential and vertical sections xis, prepared from the original of flg. 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 xso to show the minute structure more clearly . 



16. Central part of a transverse section of same specimen, xis. 



17 and 18. Vertical and tangential sections xis, of a specimen with two series of cystiph- 

 ragms, thinner walls, much larger maculae, and more abundant acanthopores than the 

 preceding. 



19. Portion of one of the clusters of mesopores xso. 



20. Several zooecia of the same tangential section xso. 



Middle third of the Trenton shales, St. Paul and Minneapolis. 



Figs. 21 and 22. HOMOTRYPELLA MULTIPORATA, n. sp 230 



21. Tangential section xis, showing, among other peculiarities of the species, the abun- 



dance of the mesopores. 



22. Peripheral part of a vertical section xig, showing the comparatively remote tabulation 



of the tubes. 

 Middle third of the Trenton shales, St. Paul. 



Figs. 23 to 30. HOMOTRYPELLA (?)OVATA, n sp 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 xis. 

 28 and 29. Vertical and tangential sections xis. 



30. One zocecium and adjacent parts of the tangential section, xso. 

 Galena shales near Cannon Falls, Minn. 



Figs. 31 to 33. HOMOTRYPELLA KUSTICA, n. sp 234 



31. Vertical section xig. 



32. Two portions of a tangential section xi8, 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. 



