PLATE XXVn— Continued. 



SriCTOroRA Gilbekti. 



Page 90. 

 See Plate 28. 

 Fig. 20. A fragment of a frond, natural size. 



Fig. 21. An enlargement of the adherent surface of the cells, the mesotheca having been removed. (X 6.) 

 Fig. 22. A transveree section, showing the form of the branch and the upright portion of the cell tulies. (X ti.) 

 Fig. 23. The lower portion of the figure (a), showing the natural surface of the frond, and on the left 



hand, the part adjacent (6), shows the cell tubes, the outer covering having been broken 



away ; the upper right hand comer (c), shows the inner jispect of the opposite face. (X 6.) 

 Fig. 24. A longitudinal natural section enlarged, showing the form and manner of growth of the cell tubes, 



the strongly annulated appearance caused by the intercellular vesicular structure, and the 



striations on the interior of the cell-walls formed in the courae of growth by the lunate denti- 



culation of the apertui-e. (X 6.) 

 Fig. 25. A transverse section enlarged, .showing the form of the branch and the interior chai-acter of the 



ridges separating the ranges of apei-tures. (X 6.) 

 Fig. 26. One of the cells greatly enlarged, showing moi>e distinctly the character of the lunate denticula- 



tion. (X 40.) 

 ¥ig. 27. A transvei-se section enlai-ged to six diametei-s. 



SW. 28. An enlargement of the surface of a specimen, which has been much macerated, the cell apertures 

 ' " ' having the appearance of the fenestrules of some fonn of Feiiegtella, and the intervening 8i)ace 



resembling the branches and dissepiments. (X 6.) 

 Fig. 29. The inner portion of the coencecium eidarged, showing the in-egular vesiculose structure between 



the cells. (X 6.) 

 Fig. 30. An enlargement of a portion of the sui-face of a branch, showing the strong lunat« denticulations, 



the extremities of which in the process of growth, form the parallel striations along the intei'ior 



of the cell tube. (X 6.) 

 Fig. 31. An enlargement of a portion of the surface of a branch near the margin, where the cell apertures 



are pustuliform. (X 6.) 

 Fig. 32. An enlargement of a portion of a branch where the exterior has been removed, showing the form 



and arrangement of the basal portion of the cell lubes as they rise from the mesotheca. 

 JP\g. 33- A portion of the surface of a froml enlarged, showing strong denticulations and a sharp depression 



along thu midiUe of the ridge .separating the ranges of cell aperatures. (X 6.) 

 Fig. 34. An enlargement of a portion of a frond near the spreading base. (X •)■) 

 Fig. 35. The mesotheca enlarged, showing the radiating longitudinal lines, which mark the direction of 



the cell tubes, iand the transverse arching undulations of growth. (X 6.) 

 Upper Helderberg group. Iialh of the Ohio river. 



