506 MR. 0. T. JONES ON THE HARTFELL-VALENIIAN []NoV. I909, 



several forms related to Climacograptus scalaris, being the most 

 abundant. In the lower part of the Bheidol Group, also, Diplo- 

 graptidse are the only forms, represented chiefly by Climacograptus 

 scalaris var. normalis, CI. medius, and CI. rectangular is ; but about 

 150 feet up the first Monograptus (ill. atavus) makes its appearance, 

 in company with Dimorphograptus and Climacograptus tornquisti. 

 M. atavus has, however, a considerable vertical range above the 

 horizon at which it first appears, although it is there accompanied 

 by other forms of Monograptus which are not represented at lower 

 horizons. 



The zone of Monograptus rheidolensis is especially characterized 

 by the abundance of Climacograptus tornquisti, which is more abun- 

 dant than the zone-form itself, but is not confined to this zone. In 

 this and the succeeding zone of M. cyphus, the well-marked species 

 Orthograptus mutahilis is extremely abundant at certain horizons. 



It is a striking fact that iu these zones, and even in the suc- 

 ceeding zone of Monograptus communis, there is a total absence of 

 straight rigid Monograpti ; every species is more or less curved. 

 In the zone of M. communis, species with triangular thecse first make 

 their appearance; they become important in the Castell Group. Also, 

 towards the middle of the zone, the genus Rastrites appears for the 

 first time in the ascending sequence, and is represented by several 

 forms. The fauna of the leptotheca band is interesting as a con- 

 necting link between those of the Rheidol and the Castell Groups ; 

 at that horizon species of Monograptus carrying thecse with well- 

 developed lobes are fairly common, foreshadowing the dominant 

 types of the succeeding group. 



In the Castell Group coiled forms with triangular thecse (M. con- 

 volutus, M. decipiens, M. cf. involutus, etc.), and more or less straight 

 forms having strongly lobed thecse (M. lobiferus, M. harpago, etc.), 

 may be regarded as the characteristic types ; but they are accom- 

 panied by straight rigid Monograpti with simple cells (M. regularis, 

 M. cf. nuclus, etc.). In the higher beds, forms having more 

 complicated thecse {M . cf. runcinatus, M. sedgwicki, etc.) occur, 

 representing early mutations of forms which characterize more 

 especially the overlying Ystwyth Stage. Thus it is possible, 

 by paying attention to the general types of species, to determine 

 the main groups of the Stage, even though the forms may not 

 be specifically identifiable. This consideration is likely to be of 

 importance in mapping certain parts of the Central Welsh region, 

 where intense cleavage will probably render the precise determin- 

 ation of the fossils a matter of considerable difficulty. 



C. The Ystwyth Stage. 



The highest stage in the district is of some importance, on account 

 of the large area which it occupies. This is due, not only to the 

 considerable thickness of the rocks included within it, but also to 



