PLATE XX. 



LOWER TREMADOC SLATES AND ARENIG GROUP. 



ARENIG ROCKS. 



Figs. 1, 2. lUcenopsis Thomsoni, Salter. From the mine-works at Ritton Castle, 



Shelve, Shropshire. (Mus. Pract. Geology.) 



lower TREMADOC SLATES. 



Figs. 3 — 12. Niobe Homfrayi, Salter. Penmorfa Church, near Tremadoc. (In 



the cabinet of Mr. David Homfray.) 



3. „ „ Nearly full-grown, the free cheeks only wanting. 



4. „ „ Head, showing more or less completely the glabella- 



furrows. 



5. „ „ Tail, showing the caudal fascia well. (My own 



cabinet.) 



6. „ „ Complete head; the free cheeks obtuse, not 



pointed. (Probably c^* •) 



7. ,, „ Young specimen ( $ form), the general shape 



and the axis being narrower than usual. 

 8 — 11. „ „ Outline diagrams from actual specimens, showing 



the true position of the glabella-furrows. 

 12. „ ,, Broad ( ? ) form, compressed and shortened by 



cleavage. 

 Figs. 13 — 19. Psilocephalus innoiatus, Salter. Specimens of all ages and various 



degrees of compression in the rock. Fig. 16 

 is the fry ; but the metamorphosis has not yet 

 been properly observed. 



