DIPLOGRAPTUS. 227 



A flattened tube, which probably contains the virgula, is a conspicuous feature of 

 the polypary beyond its distal extremity. 



Horizon and Locality. — Hartfell Shales (zone of Dicranog. Glingani). 



8. Scotland : Hartfell. 



Collections. — Lap worth, and Geological Survey of Scotland. 



Diplograptus (Orthograptus) Whitfieldi (Hall). Plate XXVI II, figs. 6 a— d. 



1859. Graptolithus Whitfieldi, Hall, Pal. New York, vol iii, p. 516, figs. 1, 2. 



1867. Diplograpsus Whitfieldi, Nicholson, Geol. M:y., vol. iv, p. 3, pi. vii, figs. 4, 4a. 



1868. Diplograpsus Whitfieldi, Carruthers, Geol. Mag-., vol. v, p. 131, pi. v, figs. 3 6, 3 c. 



1876. Diplograptus Whitfieldi, Lapworth, Cat. West. Scott. Foss., pi. ii, fig. 45. 



1877. Diplograptus Whitfieldi, Lapworth, Grapt. Co. Down, p. 134, pi. vi, fig. 21. 



Polypary short, not exceeding 2 cm. in length and 3 mm. in breadth (exclusive 

 of spines), usually both shorter and somewhat narrower. Sicula 1 mm. long, 

 generally obscure, virgella conspicuous. Thecse twelve in 10 mm., having 

 a length of 1*5 — 2 mm. and overlapping one third to one half their extent. 

 Apertural margins sub-lobate, bearing slender, stiff spines, in most cases 

 directed upward. 



Description. — The polypary varies considerably in its dimensions with age, but 



Figs. 149 a and b.-Orthograptus Whit- is illwa J S sll0rt aild datively broad ill proportion 



Heidi (Hail). ^ ^ s l en gth • 011 tlie whole, the specimens of 



commonest occurrence average 1'5 cm. in length 

 and about 2 mm. in breadth ; larger specimens are 

 rare. 



Th. I 1 originates basally, and th. I 2 crosses the 

 sicula almost horizontally so that the proximal end 

 has a very symmetrical appearance. The virgella 

 is usually long (4 mm.), and the two basal thecse 

 have apertural spines, while th. I 1 appears to have a 

 "' mesial one in addition. The thecse are of the 



a. Proximal end, showing long virgella. _ . m-j.ii 



Enlargement of part of Pi. xxviii, general Orthograptus type; m profile view they nave 

 6. Distal thecse, showing apertural an average inclination of about 30°. The apertural 



g^ eS Eniargement of part ^/"pl margins are sub-lobate, and from their outer angles 



xxviii, %. g c. t | ie s ] en( j er s tiif spines project ; these are commonly 



about 1 mm. in length, but in very well preserved specimens they have been 

 noted as having a length of 2 mm. or even more; they are usually straight and 

 have an upward and outward tendency, though this is not invariable. 



Affinities. — Orthograptus Whitfieldi has certain affinities with all the other spinose 

 Orthograpti. It should, however, be easily distinguished from them by the stiff, 



