Wyatt-Edgell — The Aremg and Llandeilo Groups. 113 



EXPLANATION OF PLATE VII. 



Fig. 1. Didymograpsicsjlaccidus {lS.aR.s^.),n9.t.s\ze. 

 1 a. Portion of the stipe of the same enlarged. 



2. Portion of D. Jlaecidus, showing the radicle, with three small spines on the 



opposite margin. 



3. Another specimen, with a longer radicle. 



4. Biplograpsus Whitfieldi (Hall, sp.), nat. size. 



4 a. The same, enlarged. 



5. Biplograpsus nmcronatus (Hall, sp.), nat. size. Introduced for comparison 



with D. Whitfieldi. 



5 a. Portion of the same, enlarged. 



6. Biplograpsus quadri-mucronalus (Hall, sp.), nat. size. 



6 a. Portion of the same, enlarged. 



7. Enlargement of a poi'tion of B. quadri-mucronatus, after Hall, showing 



the two spines arising from each cellule. 



8. Transverse section of the stipe of D. quadri-mucronatus., after Hall. 



9. Corijnoides calicularis (gen. nov.), nat. size. 9 a. The same, enlarged. 

 10. Ditto, with a single mucro. 11 . Cup of Cbr!/wo!cf<;s, enlarged. 



12. Ih'plograpstis ttibular if ormis {n. s-p.), nat. size. 12 a. The same, magnified. 



13, and 13 a. Ditto, in which the lateral cellules are reduced in number. (Fig. 



13) nat. size, and (Fig. 13 «) enlarged. 



14 and 14 a. Bounded variety of D. tubularifonnis, in which the lateral cellules 



are still further reduced in number. (14) nat. size, and (14 a) 

 enlarged. 



15 and 15 a. Another variety of the same, in which only the terminal cellules 



are left, nat. size, and enlarged. 



16 and 16 a. Biplograpsus acuminatus, (n. sp.), ordinary form, nat. size and 



enlarged. 16 5. Portion of stipe of the same, enlarged. 16 c. 

 Radicle of the same, enlarged. 



17 and 17 a. Variety of B. acuminatus, in which the cellules are rounded, and 



very variable in shape, {D. angustifolius Hall. .•") 17 b. Portion 

 of the stipe of the same, enlarged. 

 18. Didymogr apsus anceps (n. sp.), with rudimentary radicle, nat. size. 

 18 fl. Portion of the same, enlarged. 

 19 and 20. Ditto, without any apparent radicle, nat. size. 20 a. Portion of 

 stipe, enlarged. 



21 and 21 a and b. Germs of Biplograpsus pristis, enlarged. 



22 and 23. Gei-ms of a di-prionidian Graptolite, enlarged. 



24. Germ of a mono-prionidian Graptolite, enlarged. 



V. — On the Arenig and Llandeilo Groups.^ 

 By the late H. Wyatt-Edgell, Esq., 13th Light Infantry. 



THE lower strata of the Silurian system are, as yet, but little 

 known. The labours of Welsh and other geologists have 

 brought to light the large fauna of the Lingula flags ; but while 

 the Upper Cambrian has thus been established and elucidated, the 

 beds which lie immediately above it, namely the " Arenig group," 

 and the Llandeilo flags, remain to be searched, and a great part of 

 the respective fauna of each to be described. 



The Arenig, or Skiddaw, group of Sedgwick forms the base of the 

 true Silurian rocks ; it immediately underlies the Llandeilo flags, 



* This paper (originally read before the " Geologists Association," and published 

 in their Proceedings in July, 1866,) was handed to the Editor of the Geological 

 Magazine, in the autumn of last year, by the Rev. E. Wyatt-Edgell, with a request 

 from his son (at that time in Ireland) that it should appear in this Journal. In 

 addition to the author's own MS. corrections, the copy was obligingly corrected by 

 Mr. J. W. Salter, F.G.S.— Edit. 



VOL. IV. — NO. XXXIII. 8 



