ANNIVERSARY ADDRESS OF TIIE PRESIDENT. 



137 



group up to the close of the Tremadoc the genus Orthis possesses 

 (so far as we know) only 4 species, viz. Orthis Hicksii, 0. Carausii, 

 0. lenticularis, and 0. Menapice ; in the Arenig 10 species, the 3 

 last-named forms being in common with the horizon below ; in the 

 Llandeilo 13 species, 4 of which are Arenig also, viz. Orthis alata, 

 0. calligramma, 0. remota, and 0. striatula. 



In the Caradoc and Bala group the species have increased to no 

 less than 41, all having extensive geographical distribution ; 7 are 

 peculiarly Irish. In the Lower Llandovery we know of 20 species, 

 17 of which are Caradoc, the only 3 peculiar Lower Llandovery 

 forms being Orthis Bouchardi, 0. reversa, and 0. mullochensis. Thus 

 through this one genus alone we see the close alliance of the so- 

 called Lower Llandovery (the Upper Bala of Sedgwick) with the 

 Caradoc. The moderate depth at which they lived is clearly indicated 

 by the coarse and varied arenaceous nature of the deposits, for there 

 is no evidence to show, even by the zoological grouping, deep-sea 

 conditions. No Lingular are known in the Lower Llandovery rocks ; 

 although 6 species occur in the Caradoc and 5 in the Upper Llan- 

 dovery. The fauna generally may be regarded as one accumulating 

 under decreasing depth or slow elevation over given areas. 8 of the 

 20 Lower Llandovery species pass to the Upper Llandovery or May- 

 Hill beds. To show still further the decline in specific as well as 

 individual members of the genus Orthis, I may mention that in the 

 Upper Llandovery there are 10 species, but not a single form 

 belongs truly to that horizon ; Orthis rustica, the only true Upper 

 Llandovery species, passes to the Wenlock rocks, so that no form of 

 Orthis is special to the Upper Llandovery. The species of Orthis in 

 the Wenlock rocks number 16, of which 8 come from the Llan- 

 dovery and Caradoc and 4 pass to the Lower Ludlow (Orthis biloba, 

 0. crassa, 0. elegantula, and 0. hybrida), so that Orthis Edgeliana, 

 Salt., 0. Lewisii, Dav., 0. Hughesii, Dav., and 0. mullochensis , var., 

 or variety of 0. reversa, are new and restricted forms. None pass 

 to the Devonian. I have selected this genus in the Caradoc for com- 

 parison with the other Silurian groups on account of its magnitude 

 and stratigraphical value. Two other genera, Leptcena and Stropho- 

 mena, which first appear in the Llandeilo, also characterize the Cara- 

 doc by greatly increased specific development. These are Leptoena 

 with 4 species and JStrophomena with 2, both individually numerous 

 in North and South Wales, In the Caradoc Leptcena yields 8 species 

 and Strophomena 19, and for the first time we meet with Bhyncho- 

 nella with 8 species ; but no form is known to occur in South Wales, 

 and only 3 have been found in North Wales. Ireland and Scotland 

 yield 7 of the 8. The following Table shows the specific and stra- 

 tigraphical value of these four important genera from the Cambrian 

 to the Upper Silurian : — ■ 



