MONOGRAPTUS. 



405 



Figs. 271 a — d. — Mnnograptus Sander- 

 soni, Lapworth. 



Description. — The polypary is always slender, and shows ventral cnrvature 

 especially in the proximal portion, in which it is very prononnced in some individnals 

 (see Plate XXXIX, fig. 10 Ij). Distally the amonnt of cnrvature tends to decrease 

 as a general rule, and fragments appear practically straight. 



Owing to the extreme tenuity of the proximal end it is difficult to find the 



sicula preserved, but it appears to be at least 1"G 

 mm. in length, and to form the proximal termina- 

 tion of the polypary (Fig. 271 a). 



The thecte are characteristic ; they resemble 

 very closely those regarded as typical of the genus 

 Leptograptiis, with sigmoid curvature of their walls 

 and introversion of the apertural margins ; they 

 are commonly 2*5 mm. in length, and overlap one- 

 half their leng^th as a maximum. 



Affinities. — In the characters of the tliecae and 

 general tenuity of the polypary Monog. Sandersonl 

 approaches M. incommodns, but differs markedly in 

 the curvature of the polypary. The two species may 

 readily be distinguished except in small distal frag- 

 ments, which are very similar. 



Horizon and Localities. — Llandovery (Birkhill 

 Shales), especially zone of M. cijphus. 



iS. Scotland : Waterfall, Long Linn, Dobb's Linn ; 

 Frenchland Burn; Belcraig Burn, etc. Lake District: 

 Skelgill, etc. Ireland. .- Coalpit Bay, Donaghadee. 

 Wcdes : East bank of road, 50 yds. E.N.E. of Bryn- 

 chwitli Farm House, Pont Erwyd ; Rheidol Gorge, 

 400 yds. E.S.E. of same, Pont Erwyd, etc. 



Associates, etc. — Monog. Sandersoni occurs in 

 Scotland in some abundance in the zone of M. 

 ci/plius, associated with M. cgphus, M. revolutus, M. 

 atavns, M. incommodns, Dimorpliog. phjsophora, Climacog. medins and other forms. 

 It occurs on a similar hoi-izon in the Lake District in the Dimorpliog. confertus band 

 associated with M. incommodns and M. atavns; and also in Central Wales and 

 Ireland. In S. Scotland, Ireland and C. Wales it also ranges up into the lower beds 

 of the succeeding zone of M. gregarius, where it is associated with M. finihriatus, 

 M. gregarius, M. triangnlatus var., Climacog. Tornquisti and other forms. 



(7o^/^c^/'o».s.— Geological Survey of England and Wales, Sedgwick Museum, 

 Lapworth, Jones, and the Authors. 



a. Proximal portion, showing the siciila 

 in part. On same slab as PI. XXXIX, 

 fig. 10 c. 



h. Proximal theca3, showing various 

 views of apertural margins. Dobb's 

 Linn ; Birkhill Shales. Coll. Lap- 

 worth. 



e. Distal theciE, showing introverted 

 apertural margins. E. bank of road 

 from Pont Erwj'd to Devil's Bridge; 

 Castell Group. Geol. Survey ot 

 England and Wales. 



d. Distal thecoe, slightly different view. 

 Enlargement of part of reverse of 

 specimen figured ou Plate XXXIX, 

 fig. 10 e. 



