386 



BRTTTSH aRAPTOLlTES. 



In addition to tlio typical form aljovo described tliere appears commonly on 

 the same horizon a form in which tlie })olyparj is not recnrved, bnt is Ijroadly 

 arcnate throughont its length. This Tcirnquist has separated as var. (nisfertts. 



Tjocaliiicx. — N. Scotia ml: Dobb's Linn. N. Wales: Llanystwmdwy. Jrclaud : 

 Coalpit Bay, Donaghadee. 



Coll ecf ions. — Jones, Fearn sides, and the Anthors, etc. 



Fias. 255 a — c. — Monogniptuf revolutns, 

 vfir. pcaTiirsoc, var. nov. 



Var. praecursor, var. nov. Plate XXXVII I, figs. 



3 a, h. 



a. Thecse of proximal portion. Llanys- 

 twimlwy, near Criccieth ; Llando- 

 very Beds. Coll. Fearnsides. 



h. Proximal tlieca;, showing marked 

 retroversion of apertural rof^ion. 

 Enlargement of part of PI. 

 XXXVIII, fig. 3 a. 



c. Distal thecffi of same specimen. 



Ill addition to var. ansterus tliere is also found 

 another variety of Monog. reoolatas, which in some 

 respects may be regarded as intermediate in 

 character between M. revolutns and M. dijf'oi'tiiis. 

 In the form of the polypaiy it agrees with M. rcvo- 

 hitiifi, bnt the type of thecfe shows a greater degree 

 of approximation to that of M. dijf'onriis. The 

 thecal walls show a decided tendency to ogee curva- 

 ture, and the tendency to retroversion affects not 

 only the apertural margin but also much of the 

 apertural region, so that nearly one-third of the 

 theca becomes free, or technically speaking, isolate. 

 The hook-shaped type of theca thus originated 

 becomes common and characteristic in graptolites 

 belonging to higher graptolitic horizons and seems 

 to culminate in the form known as M. priodon. 

 This variety may therefore appropriately be termed 

 var. prmcnrsor. 



Localitij. — 0. Wales -. Rheidol Gorge. iV^. Wales : 

 Llanystwmd wy. S. Scotland. : Dobb's Linn (?). 



Collections. — Geological Survey of England and 

 Wales, Jones, Fearnsides, and Lap worth (?). 



Monograptus difformis, Toniquist. Plate XXXVII F, figs. 4 a— ;i. 



1899. Moyiograptua (Jiffurnih, Tuinquist, Liiiids Univ. Av.ssk., vol x.kxv, pt. 2, no. 1, p. 13, pi. 

 ii, fii?s. 1.5—17. 



Polypary several cm. in length, arcuate distally, with involute proximal portion, 

 Avidening gradually till a niaximiiui Iji-cadtli of al)out 1 mm. is attained. 



