Vol. 65.] SUCCESSION ABOUND PLTNLIJION AND PONT ERWYT). 489 



(F. 16) : — Rastrites approximatus, R. longispinus, Monograptus 

 atavus, M. communis (c), M. concinnus, M. Jimbriatus, M. cf. 

 Jimbriatus, M. gregarius, M. revolutus (?), M. triangulatus (c.) 

 and var., Orthograptus cf. bellulus, 0. insectiformis (c), Glypto- 

 graptus sinuatus, Gl. tamariscus, Climacograptus hughesi, and CI. 

 t'drnquisti (v.c). 



The genus Rastrites makes its first appearance in the ascending 

 sequence, and the usually rare form Orthograptus insectiformis occurs 

 here commonly. Monograptus revolutus, so abundant on lower 

 horizons, has almost if not quite disappeared ; while other species 

 which occur with it there linger on into this band. The typical 

 form of Monograptus triangulatus is abundant, while the variety 

 mentioned above occurs less commonly. The graptolites are mostly 

 preserved in full relief, and many of the specimens are very 

 beautiful. 



The thin band of dark mudstones (6) is distinguished by the 

 abundance of Mesograptus magnus, which makes its first appearance 

 at this level (F. 17). This species was described by Dr. Herbert 

 Lapworth from an exactly corresponding position iu the lihayader 

 •district. In the Rheidol section it is associated with the following 

 forms : — Monograptus argutus (?), M. attenuatus, M. concinnus (?), 

 M. communis var., M. cf. millipedia, M. cf. triangulatus, Rastrites 

 approximatus, R. longispinus, and Climacograptus t'drnquisti. 



The form referred to Monograptus communis occurs as abundantly 

 •as Mesograptus magnus, and appears to be restricted to this horizon 

 and the overlying leptotheca band ; it is different from the M. com- 

 munis of band No. 4. 



The flaggy beds succeeding the magnus band become progressively 

 paler in colour upwards, and approximate in their lithological 

 characters to the main mass of mudstones which conclude the 

 section at this locality. The highest graptolitic band (8) occurs 

 in these pale mudstones, and is perhaps the most interesting bed 

 in the whole district, on account both of the beauty and variety 

 •of its graptolites and of its extraordinary persistence with the same 

 characters throughout the district. The following are the forms 

 identified from this locality. (F. 18) : — Monograptus cf. argenteus, 

 M. argutus (c), M. communis var., M. concinnus, M. cygneus, 

 M. gregarius, M. inopinus, M. leptotheca (a), M. cf. millipedia 

 (v.c), M. cf. minis, M. revolutus (?), M. cf. urceolus, Mesograptus 

 magnus, Glyptograptus tamariscus, Petalograptus palmeus and var. 

 tenuis, Climacograptus hughesi (v.c), and CI. normalis (?). 



The affinities of this band, which may be designated the 

 leptotheca band, are on the whole with the underlying beds, 

 and I have drawn the base of the succeeding group above it. 



Near the centre of the dark-blue mudstones occurs a thin flag 

 (about three quarters of an inch) of a very pale rock with greenish 

 stripes; it resembles somewhat the main mass of pale mudstones 

 which succeed the graptolitic band, but is decidedly lighter in colour 

 and has a peculiar greenish tinge. It has been found in this position 



