MONOGRAPTUS. 



185 



Figs. 254 a and h. — Monograptus 

 revolutiis, Kxirck. 



extremity of the apertiiral margin. This character is most pi-oiiounced close to 

 the proximal end where the thecee barely overlap at all, and where the appearance 



presented is almost as if the denticle were prolonged 

 into a definite tag or rostel (rostellate). With the 

 increase in length of the thecge and their increase 

 in the amount of overlap, the tendency to retro- 

 version becomes less and less and the apertural 

 margin is merely somewhat rounded and l)eak-like 

 {rostf(ifc). Finally this tendency disappears, leaving 

 the thecae for the remainder of the length of the 

 polypary with even and slightly everted apertural 

 margins. The number of theca3 that show this 

 tendency to retroversion varies very much in diffe- 

 I 'v\ rent specimens, but it is always shown by a con- 



I '\\ siderable number. 



I \\ Affinitieii. — Tn the form of the polypary Monog. 



\ \ rn'oliifiis comes closest to M. cuphna, but may be 



readily distinguished from that species by its more 

 a. Extreme proximal portion. Rhei- slcuder proxiuial end and by the character of the 



dol Gorge, Cardiganshire; Rheidol .,,„.. 



Group (zone of M. cyphiis). Coll. proximal tlicc^ ; trom its variety var. praeciirsor it 

 h. Somewha"7uore distal thec». On differs in the character of the proximal theca3, which 



same slab as fig. 254 a. 4^ 1 f f rl 



Note.-As the form of the whole poly- ^^'^ ^^^^ ^^SS retrOVCrtea. 



CU^h?;ma;^Tef^li:p"ot ^^orkon and Localiiies.~U^^,doyery (Birkhill 



sibiy to the succeecUno- variety- gbales), zoucs of M. cyphus and M. (irec/aviu^. 



S. Scotland: Waterfall, Long Linn, Dobb's 

 Linn; Garple Linn; Frenchland Burn. C. Wades: Rheidol Gorge, 400 yards 

 E.S.E. Bryn-chwith Farm, Pont Erwyd; 235 yards S. of Bryn-chwith Farm, 

 etc. N. Wales: Llanystwmdwy, near Criccieth. Lake District : Skelgilh 

 Ireland: Side Valley, Mullaghnabuoyagh, Pomeroy; Coalpit Bay, Donaghadee. 



Associates, etc. — M. rcvolntas is a fairly abundant fossil in the beds at the base 

 of the zone of M. grejarius, and it also ranges up into that zone. It is commonly 

 associated with Monog. cyyltus, M. atavus, M. Sandersoui, B'lmorpJiog. physophora, 

 D. confertus, Climacog. rectavgularis at the lower horizon, and with Monog. gregarias, 

 M. fimbriatus, Climacog. Tornqvisti in the zone of M. gregarius. 



Collections. — Geological Surveys of England and Wales, and Scotland, Jones, 

 Fearnsides, Lapworth, and the Authors. 



Var. austerus, Tornquist. Plate XXXVIII, figs. 2 a- 



1899. Mono(jraj:ttus revohdus, var. austerus, Tornquist, Mouograptiiloo of Scaiiian liastrites Beds, 

 Limds Uiiiv. Arssk., vol. xxxv, pt. 2, no. 1, p. 12, pi. ii, figs. 12—14. 



