Vol. 52.] FAUNA OF THE KEISLEY LIMESTONE. 437 



between the five or six pleurae. By careful inspection, however, it 

 is seen that a terminal portion, consisting of two or three segments, 

 is bent down rather more sharply than the rest, as if composed of 

 one piece. This is the pygidium, with a very short axis — about one 

 half its entire length — crossed by two or three faint rings. The 

 breadth of the pygidium is about three times as great as the length. 



EXPLANATION OF PLATES XX. & XXI. 

 Plate XX. 



Fig. 1. Illanus galeatas, sp. n. From above. Natural size. 



2. Do. From the side. Natural size. 



3. Do. Another specimen. From behind. Natural size. 



4. Ill anus Bowmani, var. brevicapitatus. Natural size. 



5. /. Bowmani, var. longicapitatus. Natural size. 



6. Illcsnus, sp Hypostome. Natural size. 



7. Cheiriirus keislsyensis, sp. n. Natural size. 



8. Do. Pygidium. Natural size. 



9. Do. Hypostome. Natural size. 



10. Ch. (Pseifdosphcsrexochtcs) subquadratus, sp. n. Natural size. 



11. Do. Another specimen. x2. Showing fixed cheek. 



12. Sphcerexochus latirugatus, sp. n. x 2. 



Plate XXI. 



Fig. 1. Ampyx binodidosus, sp. n. X 2. 



2. Cyphaspis? Harknessi, sp. n. x2. 



3. C. {Tdrnqicistia, subgen. nov.) Nicholsoni, sp. n. x4. 

 3a. Do. Probably free cheek. X 3. 



4. Harpes, sp. a. x4. 



5. Do., sp. j3. x4. 



6. Acidaspis convexa, sp. n. X 6. 



7. Lichas bifurcatus, sp. n. X2. 



8. L. bulbiceps, Phill. MS. From above. X2. 

 8a. Do. From behind. X 2. 



8b. Do. From the side. x2. 



9. Do.? Hypostome. X3. 



10. L. conformis, Ang. var. keisleyensis nov. X 1^. 



Discussion. 



The President congratulated the Author on his important dis- 

 covery, which, he said, proved that it was yet possible to add greatly 

 to faunas in comparatively well-known rocks. He asked whether it 

 was not possible that some of the smaller forms referred to were 

 merely young specimens of the large species. 



Mr. R. S. Herries enquired as to the horizon of the Keisley 

 Limestone. 



Mr. Marr also spoke. 



The Author, in reply, explained that he had purposely employed 

 the name of 'Keisley Limestone' for the rock from which the 

 fossils had been obtained, because the question of its exact strati- 

 graphical horizon and of that of its much-disputed equivalents else- 

 where could not be discussed until its whole fauna had been de- 

 scribed. He could, however, by anticipation, say that the affinities 

 of its fauna were with that of the higher beds of the Ordovician, 

 and not with that of the Silurian. 



