1.04 



BK1TI8H GRArTOLlTKS. 



Figs. 270 a—c. — Monoijroptus atavus, Jones. 



^ 



k 



Aljliiilics. — Moiiiuj. ((f((rns conies, })c'i'luii)S, closest to M. (riiuis, Portlock, with 

 wliicli sj)ecies it was coufouiided until recently. The theca) of Mhikhj. ((tavii.s are, 



however, shorter and more closely set, and 

 also inclined at a higher angle ; the apertural 

 margins, too, are even and devoid of the 

 denticles or rostels frequentl}^ found in M. 

 tenuis. Monog. afaviis also shows some re- 

 semblance to M. (ii-fjiitiis ; but the thecal 

 walls are less curved, and the apertural 

 margins are not introtorted. 



Horizon and Localities. — Llandovery 

 (Birkhill Shales), zones of Orthog. vesictilosiis, 

 M. cijphus and M. gregarms. 



8. Scotland : Dobb's Linn; Belcraig Burn. 

 Ireland : Coalpit Bay, Donaghadee ; Mill 

 Sluice below Slate Quarry, Pomeroy. Lake 



'^ 



I 



A 



i 



11 



c 



a. Proximal fra<^iiient, showing sicula almost 



complete. Eheidol Gorge, Cardiganshire ; 



Rheidol Group (zone of M. atavus). Coll. 



Jones. 

 6. Proximal portion, showing part of sicula. Ibid. 



c. Proximal portion in relief, showing sinuous 



character of thecae. Figured, Quart. Journ. _,^ . . rn i -n r^ -n iir 



Geol. Soc, vol. Ixv, p. 533, fig. 1« !/. Rheidol Distvict : bkelglll ; bpeUglll. H (at'6' : itliay 



Gorge. Coll. Geol. Survey of England and 



Wales (No. 23710). 



d. More distal thecae ; part in relief, part cast. 



Ibid. 



e. Ibid. Part of Fig. 18c (No. 23707). 



ader ; Rheidol Gorge, Pont Erwyd ; Pary's 

 Mountain, Anglesea ; Llanystwmdwy. 



Associates, etc. — Monog. atavus is the first 

 Moiiograptiis to make its appearance in geological time. In S. Scotland it appears 

 considerably below the base of the M. ci/pJiiis zone in the zone of Orthog. vesiciilosus, 

 and ranges up into the zone of M. ci/pJins associated with M. ci/phus, M. Sandersoni, 

 and various Bi'movpliografti; at Skelgill it is found in the Diinorpliograptns band; 

 in the Pont Erwyd district also it appears in a band by itself before the coming 

 in of M. cijphus. In the succeeding zone of M. gregarias it is not so abundant, but 

 is found associated with M. fimhriatns, M. gregarias, and other forms. 



Collections. — Greological Survey of England and Wales, Sedgwick Museum, H. 

 Lapworth, Jones, Lapworth, and the Authors. 



Monograptus Sandersoni, Lapworth. Plate XXXIX, figs. 10 a — e. 



]876. MoniKjraptus Sandersoni, Liipwortb, Geol. Mag., dec. ii, vol. iii, p. 320, pi. xi, fig. 2 a — e. 



Polypary slender, of great length, with distinct ventral curvature proximally, 

 and becoming straighter distally ; Avidening almost inn)erceptibly from a 

 thread-like proximal portion to a maximum of about '7 mm. Theca3 eight 

 to six in 10 mm., very long and narrow, inchned at so low an angle that 

 their inner walls form an almost continuous line running parallel to the 

 axis of the polypary, overlapping one-half their length, and with distinctly 

 introverted apertural margins. 



