Wyatt-Edgell — The Arenig and Llandeilo Groups. 115 



it is the remarkable Calymene parvifrons, first described from Tai- 

 hirion, near Arenig-fawr, but found since then in Shropshire. 

 Triniicleus Murchisoni is from Cefn-y-Gwynlle, one of the Stiper- 

 stone range of hills. Trinucleiis Gibbsii is from Whitesand Bay. But 

 the most notable genus of this group is the singular trilobite JEglina, 

 of which there are other species besides the three here given. 

 Having but seven body-rings, no rostral shield, and a large lobe- 

 less tail, it is classed with the Asaphiis family ; but at the same 

 time the circular glabella and elongated eyes seem to connect it with 

 Bemopleurides, one of the Olenidce. It is eminently characteristic of 

 the Arenig period, where it attains its highest development ; and 

 this genus may be looked upon as one of the patriarchs of Silurian 

 trilobites, for it seems to indicate a passage from the numeroias 

 Olenidce of the Upper Cambrian to the AsapMd(z of the succeeding 

 era. Kepresented by one or two species during the formation of the 

 Llandeilo flags, this genus fades away in the Caradoc. Of shells 

 there are but few species as yet described. The best knov^Ti are the 

 Orthoceras Avelinei and Ldngida or Oholella plumbea, which are not 

 uncommon in the black shales of the Stiperstones. The latter 

 species is the only one I know which is common to the Arenig 

 and Llandeilo ; even this is doubtful, for the Obolella found at 

 Builth, and referred to this species, is not in every respect the same. 

 There is an undescribed species of Orthis found at Whitesand Bay : 

 CucuUella Anglica from the Stiperstones, the Bryozoa Bidymograpsus 

 geminus and Graptopora (Dictyonema) sp. ( ? socialis) Whitesand Bay 

 represent the inferior mollusca. Two species of Encrinites, the one 

 from Shropshire, and the other from the St. David's district, com- 

 plete the lists of the more common fossils. 



The Llandeilo series consists of two distinct strata, which I have 

 before proposed to call Upper and Lower Llandeilo flags, the latter 

 having nothing to do with the " Lower Llandeilo" of Murchison, 

 more properly distinguished by Sedgwick, as Arenig. 



It is strange that the existence of two beds making up the 

 Llandeilo flags should never have been remarked before ; for there 

 is as much difference between the subdivisions, both in fossils and 

 mineral character, as there is between the Upper and Lower 

 Llandovery, which it has been proposed to call even distinct groups 

 — not that, as a rule, the mineral character of beds is much of a 

 criterion for their classification. 



I have long thought that the true Llandeilo ought properly to be 

 subdivided into two strata, but it was not until lately that I saw 

 these occurring together, and so was enabled to determine which was 

 the higher, and which the lower member of the series. Near Builth, 

 in Eadnorshire, there appear the Upper Llandeilo flags in the shape 

 of black shales and fine white sandstone, lying conformably on the 

 Upper Llandeilo, with the associated volcanic grit and bedded trap. 

 This is seen in the section (Sheet No. 5) published by the Geological 

 Survey, which crosses the shales and bedded trap of the Lower 

 Llandeilo at the Carneddau hill, and the grit, black shales, etc., re- 

 presenting the Upper Llandeilo near Wellfield. 



