8G4 



BRlTIStl GRAPTOJATES. 



Lockerbie; Dnffkinnell. I jakr District : Skelgill. Wales: Llanystwnidwy, near 

 Criccietli; Rhayader; Pont Erwyd, etc. Trdaiid: Doiiagliadee ; Mill Sluice 

 below Slate Quarry, Pomero3^ 



Associdfes, etc.—Monog. cj/phiis is an abundant and characteristic fossil in the 

 Birkhill Shales of S. Scotland and their equivalents elsewhere, lying immediately 

 below the zone of Monog. grogarvis (sub-zone M. finihriatiis). It commonly occurs 

 associated with Monog. rerohitits and var. ansteriis, M. t^(i>idcrsoiii, M. atamis, 

 Dimorphog. jiJii/.^op/iord, Ciimocog. rccfovgiilarU, and other forms. 



Collectors. — Lapworth, Jones, Fearnsides, and the Authors. 



Figs. 237 a - d. — Monograptus acinaces, Tornq. 



Monograptus acinaces, Tornquist. Plate XXXVI, figs. 2 a—e. 



1899. Monofjraittus acinaces, Tornquist, Mouograptidae uf Scauiau Basfrites Beds, Luuds Uuiv. 



Arsskrift, vol. xxxv, pt. 2, no. 1, p. 5, pi. i, figs. 7, 8. 

 1909. Monograi^tns rheidolcnsis, Jones, Quart. Journ. Greol. Soc, vol. Ixv, p. 535, figs. 19 a — c. 



Polypary several cm. in length, approximately straight distally but arcuate in 

 proximal portion, very slender at first, Init increasing in Avidth till a maximum 



of 2 mm. is attained. Thecse six to 

 eleven in 10 mm., those of the proximal 

 portion very long slender tubes over- 

 lapping about one-half their length, 

 those of the distal portion expanding 

 somcAvhat in the apertural region and 

 overlapping two-thirds their length. 

 Dcscrijjtion. — The arcuate proximal portion 

 of the polypary Avith its very long slender 

 tliecge is highly characteristic of this species ; 

 distally the curA^ature becomes less and may 

 eventually disappear altogether. The i^oly- 

 # P^iy is of great but unknoAvn length, probably 



h reaching 80 cm. at least. Small fragments 



a. Proximal ('ii.i, showing sicuia. Figuied as [■^y whicli tlic maximum Avidth is not attained 



31. i7ic)rf()fc»is(s, (Juart. Journ. Gcol. Soc, 



vol. Ixv, fig-. 10 h. Kheidoi Gor^ro, (Jar- arc tliosc of commouest occurreucc. 



diganshire ; Rlicidol Group (zone of M. 

 rheidolcnsis). Geol. Survey of England 

 and Wales, Jerniyn St. (2370.5). 

 h. Thecse in median portion of the i)olypary. 

 Enlargement of part of PI. XXXV'l, 

 fig. 2 a. 



c. Distal thecce in relief. Rheidol Gorge ; 

 Rheidol Group. Coll. O. T. Jones. 



d. Distal theeie in relief. Figured Quart. 

 Journ. Ge(d. Soc, vol. Ixv, fig. 19 d. Geol. 

 Survey of England and Wales (23704). 



The sicula is very long and slendei', measur- 

 ing 5 mm. in length and reaching to half-Avay 

 between the apertures of tli. 1 and th. 2; it 

 alone forms the termination of the polypary, 

 since th. 1 arises some distance above the 

 aperture of the sicula. 

 The Avhole proximal end of the ]K)lypary is extremely slender, and the thecas 

 are A^ery long (4 — 5 mm.), groAving approximately parallel Avith the general 



