46S 



BRITISH GRAPTOLITES. 



Fies. 324 a and b. — Monograptus con 

 volutus (Hisinger). 



with fully four-fifths of length free, of which the outermost fifth is 



involved in reflexion to form a small inconspicuous barb, or this barb may 



appear to be more or less completely replaced by a distinct spine. 



Description. — Monog. convolutus presents its most typical appearance when 



seen as a plane flat spiral ; the mode of preservation, however, has sometimes caused 



the convolutions to be somewhat irregular in form, and at the distal end the 



whorls seem to cease and the polypary is prolonged 

 with a merely arcuate curve ; fragments showing 

 this are of fairly frequent occurrence. 



The best British specimen known to us is 



unusually large; it exhibits five whorls, but is not 



complete proximally, though it clearly shows the 



Rastrites-like character of the earlier thecge. This 



specimen has a diameter of 7 "2 cm., and the whole 



of the first whorl and half the second bear thecas of 



the Rastrites type, long linear tubes with slightly 



barbed apertural terminations, these tubes being 



twice ;is long as the interspaces which separate 



them. The theca? gradually lose their linear 



character, becoming broader in the adnate region, 



and finally are distinctly triangular in form with 



definitely spined apertural terminations, resembling 



in this respect the distal thecae of M. Stdgwichii, 



though distinctly more triangular in shape and more 



isolate. The number of thecaa in the proximal 



portion which show the Rastrites form would appear 



to vary somewhat, though there are always sufficient 



for this to be a characteristic feature of the polypary. 



Affinities. — Monog. convolutus resembles M. spiralis in its general shape and in 



the general characters of the distal thecae, but the proximal thecae with their 



Rastrites-like form are quite different, and the whole polypary is more slender 



than M. spiralis, which widens persistently throughout its extent. As regards 



the general characters of its thecae Monog. convolutus approaches M. triangulatus, 



but its convolute shape should serve to distinguish it. From M. circularise which 



it resembles in general form, it may be separated by the characters of the thecae. 



Horizon and Localities. — Llandovery (zone of M. convolutus). 



S. Scotland : Dobb's Linn ; Belcraig Burn ; Lockerbie ; Head of Pishnack 



Burn, Wee Queensbury, etc. Lake District : Skelgill ; West side Long Sleddale. 



Ireland: Coalpit Bay, Donaghadee. Wales: River Twymyn, Llanbrynmair ; 



fifty yards E. of fence, 270 yards N.E. of Gwen-ffrwd Uchaf Farm; W. and E. 



side Old Quarry, N.E. Fagwr-fawr Farm, 2 miles E.N.E. of Pont Erwyd. 



**j 



,,, i, 



a. Proximal portion, showing parts of 



two whorls. Enlargement of part 

 of PI. XLVII, fig. 1 6. 



b. Distal thecae, in relief, but somewhat 



embedded so that the apertural 

 margins are partially concealed. E. 

 side of Quarry, N.E. of Fagwr-fawr 

 Farm, Pont Erwyd. Geol. Survey 

 of England and Wales. 



