pai't 2] 



ITS STRUCTUEE AIN'D EOCK-SUCCESSIOIS^. 



163 



which, however, are not peculiar to it, but are long-ranged forms ; 

 though some, like Heterorthis alternata (Sowerby) and its variety 

 retrorsistria (Davidson), appear to be characteristic Caradocian 

 species. The trilobites, however, are more significant ; Calymene 

 planimarginata appears to range throughout, as do also Homalo- 

 notus hisulcatus and Trinucleus concentricus ; others, though 

 they may perhaps be found eventually all through the Series, are 

 at any rate more abundant in either the lower or the higher beds. 

 Thus, the large Asaplnis powisi seems to be more particularly 

 characteristic of the lower beds, as does also a form which agrees 

 well with Calymene hrevicapUata Portlock ; on the other hand, 

 species of Chasmops, Pterygometopus juhesi, Calymene caractaci, 

 and Acaste aplciilata are undoubtedl}'' more abundant in the 

 higher beds. 



Monticuliporids are abundant, similar species ranging not only 

 through the Series, but above and below its limits. Glyptocrimis 

 hasalis is everywhere a common fossil, but C3''stid-plates are more 

 numerous in the higher beds, although the}^ seem to attain a still 

 greater development in the basal beds of the Ashgillian. 



Neither gastrojDods nor lamellibranchs, though locally present 

 in fair abundance, form any important part of the fauna. 



The most satisfactory palasontological classification, therefore, is 

 that which groups trilobites and brachiopods together : the former 

 because the}'' afEord the surer guides to age, the latter because they 

 constitute so great a part of the fauna as a whole. On this basis 

 the Caradocian rocks, as developed in the Bala countrj^, may be 

 regarded as belonging to the Calymene-planimarginata fauna. 



Other fossils that range throughout in some abundance are : — 



Trinucleus concentricus Eaton, or a 



variety. 

 Hoynalonotns hisulcatus Salter, or a 



variety. 

 Orthis (PlatystrophiaJ hiforata 



(Schlotheim). 



Orthis fHarhiessellaJ vesioertilio 



(Sowerby), 

 Orthis calligramma Davidson, 

 Orthis (Dinorthis) flahellulum 



(Sowerby). 

 Plectamhonites sericea (Sowerby). 

 Monticulipora fibrosa (M'Coy). 



A further subdivision may be made into two sub-faunas : a lower 

 one characterized by the abundant presence of Asap)hus p)owisi and 

 Heterorthis alternata, and an upper containing Chasmops and 

 Orthis {Nicolella) actonice. The fossil assemblages characteristic 

 of these two sub-faunas are recorded in the lists on pp. 170-71. 

 The graptolites found are important, since they enable some definite 

 comparison to be made as to the relative ages of the beds in the 

 rocks of the * shelly 'type and ' graptolitic ' type respectively. 



The typical form of Orthograptiis truncatus Lapworth has been 

 found in the mudstones of the Glyn-Gower Beds (Pont-yr-Onen 

 and Nant-Rhyd-Wen), and, so far as our present knowledge goes, 

 tliat fossil is highly characteristic of the zone of Dicranograptus 

 clingani, although towards the top of that zone and in the 

 succeeding zone of Pleurograptus linearis, its place is gradually 

 taken by the longer and thinner vai'iet}^ paiiperatus Lapwoi'th. 



m2 



