518 MR. 0. T. JONES ON THE HAETJELL-VALENTIAN [Nov. I909, 



The Fauna of the Ystwyth Stage. 



With the exception of the annelidan jaws mentioned on p. 507 and 

 a few annelid-tracks, the fauna is entirely graptolitic. The Stage 

 as a whole, or so much of it as is represented in the area, is cha- 

 racterized by the occurrence of certain species, or groups of allied 

 species, principally of Monograptus ; but a comparison of the forms 

 from the higher with those from the lower beds shows that con- 

 siderable changes have taken place in the interval (see vertical 

 section, fig. 13, p. 517). Such changes are, however, gradual, the 

 central parts of the Stage having a fauna clearly intermediate 

 between that of the lower and that of the upper portions. 



Monograptus turriculatus, which appears suddenly in great 

 numbers in the lowest beds of the Stage, seems to have survived 

 throughout, since it is also found iu almost the highest beds; and 

 in the upper part of the Blaen Myherin Mudstones it is one of the 

 commonest forms. "With the exception, however, of a single 

 specimen from Bodcoll Quarry, about the middle of the Devil's- 

 Bridge Group, the species has not been discovered in the inter- 

 mediate beds. 



A number of forms which may be grouped around Monograptus 

 nudus occur in the Ystwyth Stage. M. nuclus var. variabilis is 

 one of the most abundant and characteristic forms in the Devil's- 

 Bridge Group, and appears to be confined to it ; the other varieties 

 which accompany it are smaller and narrower than the typical 

 form as figured by Prof. Charles Lapworth, which only becomes 

 abundant in the Myherin Group. 



Another interesting group is typified by the straight lobiferids, 

 Monograptus marri, M. Jiolmi, and M. priodon, which are common, 

 in the Myherin and Bhuddnant Groups, although the first form may 

 occur in the upper part of the Devil's Bridge Group. In the Blaen- 

 Myherin Mudstones and higher beds, M. holmi is the most abundant 

 graptolite, and is generally accompanied by forms which must be 

 referred to the early types of M. priodon ; but there appear to be 

 many gradational forms between the two species. 



Monograptus beeJei is a prominent form of the Devil's Bridge 

 Group, and appears just to survive into the succeeding group. It is 

 probably allied to M. lobiferus, which occurs in the Castell Group. 



Monograptus exiguus makes its appearance towards the top of 

 the Devil's Bridge Group, and survives at least into the top of the 

 Myherin Group, where it occurs in considerable numbers. About 

 the middle of that Group it is represented by a distinct variety, 

 which is larger than the typical species, has a more open form, and 

 more distant thecse. 1 



Among other species which have a restricted range may be men- 

 tioned Monograptus resurgens, which occurs with Rastrites linnaii 

 near the junction of the Devil's Bridge and Myherin Groups ; 



1 See also E. M. E. Wood (Shakespear), ' TheTarannon Series of Tarannon 

 Quart. Journ. Geo!. See. vol. lxii (1906) p. 679, footnote. 



