I 



PETALOGRAPTUS. 283 



The width of the polypary opposite the aperture of th. I 1 varies from 3 — 4 mm.; 

 it increases rapidly up to the level of the fourth pair of fchecse, but afterwards the 

 increase is so slight that the sides appear approximately parallel for a distance 

 proportionate to the length of the polypary ; there is, however, a certain amount 

 of variation in the width of the polypary, due to the amount of curvature of the 

 thecae ; the greater the curvature, the greater the width. 



The thecse of the central portion of the polypary are fully 7 mm. in length, 

 Fig. 195.— Petaiograptus folium (His.), but they are somewhat shorter towards the distal 



end and those developed last are quite short; the 

 average ratio between the length and breadth is 

 6 : 1> The angle at which they are inclined to the 

 median line of the polypary varies ; it is about 5° 

 at the proximal end, increasing to 20° in the median 

 portion, and decreasing again to 5° near the distal 

 end. All the thecal are concavely curved but not 

 equally so ; the curve is greatest in those developed 

 earliest, but subsequently diminishes, giving to the 

 Complete specimen, obverse aspect, w hole polvpary the characteristic foliate appearance 



Belcraig Burn, Birkhill Shales. L •' x J l *■ 



Figured, Eiies, Quart. Journ. Geoi. to which it owes its name. The thecas overlap for 



Soc, vol. liii, pi. xiii, fig. 5. Brit. 



Museum (Nat. Hist.). two thirds of their length, but this amount increases 



distally till the thecse are in contact practically for their whole extent. 



The appearance of rounding off at the distal end is due, as in Phyllogra/ptus, to 

 the decrease in curvature and diminution in length of the thecas. 



No indications of the presence of a septum have been detected in this species ; 

 the virgula takes a very irregular course, which seems to indicate that it was 

 free inside the polypary. The virgular tube is often distally prolonged for a 

 considerable distance, and is not infrequently split at one or more points along 

 its length ; it seems to have lain very near the obverse surface in the earlier part 

 of its course. 



Affinities. — P. folium is a highly characteristic species. It has long been well 

 known in Scandinavia, but in Britain it has often been confused with P. jjcdmens, 

 from which it may be distinguished by the following characteristics : (1) Its more 

 pronounced foliate form ; (2) its greater width ; (3) the more protracted nature of 

 the proximal end ; (4) the longer thecae, their lower angle of inclination, and the 

 smaller number in a given unit of length. 



Horizon and Localities. — Birkhill Shales (in the highest beds of the zone of 

 Monog. gregarius, and in the zone of Monog. convolidus). 



8. Scotland : Dobb's Linn ; Belcraig Burn ; Duffkinell Burn, etc. Lake 

 District .- Skelgill. Ireland : Coalpit Bay, Donaghadee. 



Associates, etc. — P. folium is a fairly abundant form in S. Scotland in a band of 

 shale at the top of the zone of Monog. gregarius, lying immediately below the 



