BEITISH FOSSILS. 



9 



Fig. 4. Glabella of a specimen from the Wenlock limestones of the Malverns, with the 



two upper furrows nearly obselete ; the lower ones are stonger than usual ; a 



rare variation. (Coll. Mus. Pract. Geol.) 

 Fig. 5. Under side of the head, showing the entire rostral portion a, the termination 



of the facial sutare on each outer side at hh, much further backward than on 



the upper surface (see fig. 1, h); c, the obtuse hypostome or labrum. (Coll. 



Mr. John Gray, Dudley.) 

 Fig. 6. Hypostome of last specimen, magnified. The basal processes (a) extend even 



further outwards in some specimens, and are probably attached beneath to the 



ends of the upper glabella furrows. 

 Fig. 7. Eye of an ordinary specimen, natural size. Dudley. 



Fig. 7*. Portion of do., highly magnified, showing the separate convex portions of the 



cornea over each lens, with granules on the interspaces. 

 Fig. 8. Portion of the eye of another variety, with the lenses proportionally smaller and 



more distant, and the granules collected into an hexagonal network between 



them. Dudley. 



Fig. 9. Highly magnified cast, in fine silty mud, of the interior of the eye, showing the 

 cups from which the lenses have fallen out. These cups therefore occupy the 

 place of the depressed tip of the crystalline or vitreous body. (Burmeister.) 



Fig. 10. Enlarged specimen, the head divided at the facial suture, showing the first 

 segment, a, as an entire ring or segment which bears the eyes. On the 

 second ring, b is the upper eye lobe ; c, the tubercle or rudimentary spine ; at 

 d, the pleural furrow is shown, and at e, the fulcral point of a middle thorax 

 joint ; //, the notched tips of the pleurae ; g, the tail. 



Fig. 11. Part of the front of the head and glabella, to show the equal granulation of the 

 surface. 



Fig. 12. Magnified notched ends of the pleurae (upper side), showing their surface to be 



granulated even over the facetted portion, b ; sd a the tubercle is shown, which 



is better seen in the next figure. 

 Fig. 13. Magnified under side of three pleurse, showing the narrow incurved under 



portion b, and the tubercles which serve as buttresses in rolling up, a. 

 Fig. 14. Internal cast of a large head, from Ledbury, Wenlock limestone (Mr. C. Stokes's 



cabinet) ; the furrows are much broader and deeper than usual. 

 Fig. 15. Tail, natural size, from Dudley, to show the sub -triangular pointed form usual 



in the species. 



Other British Species of Phacops, of the Section Acaste. 



1. P. apiculatus, Salter (1852), in Prof. Sedgwick's Synops. Classific. Palseozoic Rocks, 

 fasc. 2, Appendix, iii pi. 1 G. f. 17-19. Portlockia? apic. M'Coy (1851), ib. fasc. 1. 

 p. 162. 



P. omnino P. Downingice simillimus ; sed capite longiore, glabella elongatd, antice convexiore, 

 lobis basalibus circumscriptis subtriangulatis nec transversis ; sulco medio glabellari longiore, 

 supremo distinctiore ; oculis elongatis subdepressis ; angulis posticis capitis brevissime 

 mucronatis ; caudd ad apicem paullo compressd et in apiculum recurvum brevem productd ; 

 axi angustato. 



Localities. — Common in the Llandeilo flags of North Wales, and in the Caradoc sand- 

 stone of Hope Bowdler and Acton Scott, Shropshire. [Geol. Surv. and Woodw. 

 Mus.] 



Heads of this species have also occurred in the hard quartzites of the coast of Corn- 

 wall, at the Great Peraver, in company with Calymene, Orthis and other Silurian 

 forms. 



