ANNIVERSARY ADDRESS OF THE PRESIDENT. 



117 



belong to the genus Orthis : they are Crania divaricata, Siphonotreta 

 nucula, Orthis vespertilio, 0. turgida, 0. Actonice, 0. calligramma, 

 0. striatula, 0. elegantula, 0. insularis, 0. bifoveata, Strophomena 

 rhomboidalis, Leptama tenuissimistriata, and L. sericea. 



Cardiola interrupta is the only Lamellibranch out of the known 6 

 species, and Bellerophon perturbatus and B. bilobatus the only 2 Hete- 

 ropocla out of 7, which, with Favosites fibrosus, help to make up the 

 41 species connecting the North and South Wales Llandeilos ; so 

 that, of the 47 North- Wales species, 40 are common to both areas. 

 No less than 88 species occur in South Wales. The whole Llan- 

 deilo fauna for the British Islands numbers 80 genera and 175 

 species ; of these I may mention that Ireland has yielded 51 species, 

 Scotland 66, and Shropshire only 26 : these latter are chiefly from 

 the Shelve district, and are mostly Crustacea and Brachiopoda. Our 

 knowledge of the number of species occurring in Scotland is due to 

 the researches of Mr. Lapworth in the Moffat area. 



In the St.-David's area and throughout Pembrokeshire, Brecon- 

 shire, and Caermarthenshire, the typical localities of Llandeilo and 

 Builth &c. contain a special fauna. At Abereiddy Bay the black 

 slates and argillaceous limestones are interstratined with felspathic 

 tuff ; they rest upon the Arenig group, having a distinct fauna ; a 

 thick bed of tuff divides them. The immediate presence and abund- 

 ance of Didymograptus Murchisoni and Dicellograptus, Cryptograptus, 

 &c. in the slates on the south side of Abereiddy Bay above the fel- 

 spathic tuff, at once clearly marks or determines the base of the 

 Llandeilo beds. 



Dr. Hicks divides the true Llandeilo series of St. David's into 3 

 subgroups*, the lowest containing most Ehabdophora or Graptolites, 

 associated with Trinucleiis Bamsayi, Calymene Murchisonice, and 

 Theca caereesiensis, which especially characterize this lower division. 

 The middle subgroup consists of dark " calcareous shales, with 

 compact limestone at the upper part;" Asaphus tyrannus, A. pel- 

 tastes, Calymene cambrensis, Ogygia convexa, and Trinucleiis Lloydii 

 are the typical Crustacea, with Lingula granulata, Haly sites catenu- 

 latus, and 6 Graptolites. In addition to the fauna, the calcareous 

 nature of the beds lithologically separates this middle subgroup 

 from the, over- and underlying series. Everywhere it is charac- 

 terized by the forms just enumerated ; many of the same occur at 

 Llandewi Yelfrey, Lampeter Yelfrey, and Musclewick Ba;y. The 

 Upper Llandeilo is also distinguished by special forms of Trilobita 

 — Barrandia Cordai, B. longifrons, Clieirurus SedgivicJcii, Ogygia 

 BucJiii, Calymene dv/plicata, Ampyx nudus, and Agnostus M'Coyii. 

 It will be seen that each division is specialized or can be discri- 

 minated by its Crustacea ; the well-known Ogygia Buchii and 

 Clieirurus Sedgwickii, here as elsewhere, are the typical forms in 

 the upper black argillaceous slaty flaggy sandstones. The middle 

 yields Asaphus tyrannus, A. peltastes, and Ogygia convexa, &c. ; and 

 the Lower Calymene Murchisonice and Trinucleus Bamsayi; and 

 this grouping holds good wherever the Llandeilo beds occur. Three 

 * Quart. Journ. Geol. Soc. vol. xxsi. pp. 177-180. 



VOL. XXXVII. Tc 



