PLATE XVIII. 



MiCHELINA (PlEURODICTYUM) STYLOPORA. 

 Aslrea Klylopora Eaton. Geological Text Book, p. 40, pi. 4, f. 48. 1832. 



Fig. 1. Lateral view of a specimen larger than the ordinary size. 



Fig. 2. A summit view of the same individual, showing the striations of the cell-walls. Across 

 the lower part of this specimen there is a line of division indicating the existence of 

 two separate groups or twin germs in the same mass. 



Fig. 3. An enlargement of a few of the cells, showing more clearly the crenulated character of the 

 cell margins. 



Fig. 4. The base of another individual which lias commenced its growth upon the shell of a Loxo- 

 nema — a very common feature of this species in many localities. | 



In this individual the calcareous matter of the epitheca and cell-walls has been ' 

 weathered away, leaving the filling of the pores to show as connecting processes, 

 producing the condition shown in typical specimens of Pleurodictyum, the P. proble- 

 maticum being simply a weathered condition of Michelina. The details of this con- 

 dition will be shown in the Lithographic plates to be given hereafter. 



Fig. 5. View of the base of a smaller individual of similar mode of growth, where the epitheca 

 and cell-walls are preserved. 



Fig. 0. A lateral view of a specimen of very unusual form and mode of growtli, being spherical 

 and enclosing a part of a crinoidal column upon which it has grown. 



Fig. 7. A vertical section of a large individual, showing the division of cell-walls and the ori"in of 

 new cells, the irregular and distant diaphragms, which are both convex and concave, 

 and in several places the pores passing through the cell-walls. 



Fig. 8. A transverse section enlarged, showing the crenulated sides of the cell-walls, caused by the 

 remains of vertical lamellae. 



Fig. 9. A still farther enlargement of the external surface of the vertical lamella;, as shown in 

 figures 1 and 2, the surface being flattened and wrinkled. 



The specimens are from the Hamilton group, at Darien, York, shore of Lake Erie, 

 and other localities in Western New York. 



Michelina (Pleurodictyum) dividua u. s\>. 



Fig. 10. Lateral view of a specimen showing the wrinkled epitheca of the exsert individual cells. 

 Fig. 11. Summit view of a larger and very perfect specimen, showing the usual size of the cells. 

 Fig. 12. Lateral view of the same specimen (fig. 11), showing the characteristic form of the epitheca. 

 Fig. 13. Lateral view of an elevated form where the cells are but slightly divergent. 

 Fig. 14. Lateral view of another individual, which, although much elongated, does not show the 



external epitheca, the open cells covering all parts of the surface. The colony has 



been epiphytic on a convex bivalve shell. 

 Fig. 15. A vertical section of a broad flattened specimen. 

 Fig, 16, A similar view of a more elevated or upright growing form. Both the specimens show 



the closely crowded diaphragms and the large pores. 

 Fig. 17. A transverse section of a broad specimen. The figure does not show the crenulations on 



the sides of the walls covered by the striai, which in the example are very fine and 



numerous. 



