A> T XTYERSARY ADDRESS OF THE PRESIDENT. 



107 



Tai-hirion in Xorth Wales, and ranges to both Llandeilo and Caradoc ; 

 Didymograptus patulus, D. hirundo, and Clematograptus implicatus 

 are also present at St. -David's*. Few Hydrozoa occur in the Xorth- 

 Wales Arenigs ; Ty-obry, Treroadoc, and Tai-hirion yield the few 

 that have been met with. 23 species range through the St. -David's 

 series, 2 of which only are common to the Stiperstones area, viz. 

 Didymograptus patulus and D. hirundo; and 5 or 6 ally them to 

 Skiddaw. There is thus little in common between the Arenigs of 

 Skiddaw and Xorth Wales, few species connecting them. The rela- 

 tion, however, between the Skiddaw species and those of St. David's 

 is closer, 5 or 6 being Graptolites, and 2 Trilobites ( Ogygia Selwynii 

 and Trinudeus Gibbsii). These, we must remember, are the two 

 largest groups, and have been most searched for, at St. Davids 

 especially, by Dr. Hicks, and in the Skiddaw beds by Dr. A. Nichol- 

 son, A[r. Dover, and the Survey (through the late AEr. Ward). 



The value of the Arenig group is still more apparent when we 

 consider the development of life that accompanied those physical 

 conditions and changes which took place at the close of the Lingula- 

 flag and Tremadoc epochs. We know that no less than 40 genera 

 make their first appearance in the Arenig rocks in the British Islands. 

 These 40 genera belong to the following 6 classes : — 



Hydrozoa 16: Callograptus, Denclrograptus, Phyllograptus, Di- 

 dymograptus, Azy yog 'rapt us, Ptilograptus, Trigonograptus, Tetra- 

 graptus, Climacograptus, Nemagraptus, Dicellograptus, Clemato- 

 graptus, Diplograptus, Glossograptus, Dichograptus, and Logano- 

 graptus. 



AjsnsTiLLDA 4 : Helmintholithes, Stellascolites, Nereites, and Pcdceo- 

 chorda. 



Crustacea 11 : JSglina, Trinucleus, Barrandia, Oalymene, Pha- 

 cops, Placoparia, Illcenus, Illcenopsis, Homalonotus, Beyrichia, and 

 Caryocaris. 



Brachiopoda 2 : Dinobolus and Siphonotreta. 



Lamellirran-chiata 2 : Ribeiria and Redonia. 



Gasteropoda 3: Ophileta, Pleurotomaria, and Rhaphistoma. 



The succeeding Llandeilo, as we shall see, was equally prolific, no 

 fewer than 48 new genera accompanying the complete change of 

 conditions at the close of the Arenigs and commencement of the 

 Llandeilo. This change was bridged over by only 8 genera and 9 

 species passing to the Llandeilo in any area ; 4 are Hydrozoa, 2 

 Crustacea (Trinucleus Ramsayi and Homalonotus bisulcatus), 3 Bra- 

 chiopoda (Lingula brevis, L. petalon, and Orthis calligrarnma) ; and 

 only 3 range into the Caradoc. As before stated, the relation of 

 the Arenig to the Tremadoc is through 16 species, 6 of which are 

 Brachiopoda, 6 Crustacea, and 4 bivalve llollusca. Thus 122 

 species out of 150 are strictly Arenig forms or confined to that 

 horizon. 



* Fire other Skiddaw species occur west of the Stiperstones, and, with the 

 6 Graptolites, tend to connect the Arenigs of Shropshire with those of West- 

 moreland. They are the Annelida Scolithus linearis, Salt., and Helmintholithus, 

 with Agnostus Morei, Salt., Ogygia Selwynii, Salt., and JEglina binodosa. 



