DIPLOGRAPTUS. 263 



Associates, etc. — Var. compactus is a somewhat rare fossil in the Lower Hartfell 

 Shales, occurring in the zone of Dicranog. Olingani. It occurs, therefore, at a 

 higher horizon than the typical form, which does not seem to range beyond the 

 basal zone of the Hartfell Shales. 



Collections. — Lapworth and the Authors. 



Diplograptus (Mesograptus) modestus, Lapworth. Plate XXXI, figs. 11a — e. 



1876. Diplograptus modestus, Lapworth, Cat. West. Scott. Foss., pi. ii, fig. 33. 



1877. Diplograptus coufertus, Lapworth, Grapt. Co. Down, pi. vi, fig. 8. 



1897. Diplograptus modestus, Peruer, Grapt. de Boheme, pt. iii, p. 5, pi. x, fig. 8. 

 1900. Diplograptus modestus, H. Lapworth, Quart. Journ. Geol. Soc, vol. lvi, p. 135. 



Polypary robust, about 2 cm. in length, with maximum breadth of 3 mm., attained 

 by rapid widening from proximal end. Sicula small, with conspicuous 

 virgella. Thecas fourteen to twelve in 10 mm. ; sigmoid curvature 

 pronounced in both profile views in proximal region, diminishing distally ; 

 overlapping one third to one half their length ; apertural margins wide, 

 undulate, occupying about one fourth of total breadth of polypary. 

 Description. — The polypary is robust and relatively wide ; it may be said to be 



broadly rounded in the initial region, and the 

 Figs. 180 a-c— Mesograptus modestus, max i mum breadth is attained so rapidly that 



Lapw. x •> 



the margins are subparallel almost from the be- 





The sicula is small and relatively broad, and 

 measures about 1 mm. ; the virgella may be from 1 

 to 2 mm. or more in length, and is always fairly 

 stout and therefore conspicuous. Th. I 1 originates 

 a b c about half-way along the sicula, and grows first 



a. Proximal end, obverse aspect. En- downward and then upward and outward, so that 



largement of specimen on same slab . . 



as pi. xxxi, figs, ii a, b, and a. its aperture is nearly on a level with the apex of 



b. Reverse aspect. Enlargement of , .. i-iiiio , i • -, ln - i 



specimen on same slab as pi. xxxi, the sicula ; while th. 1" crosses the sicula obliquely, 

 c Proximal' end, impression of obverse so as to leave it free for a small fraction of its 



pS'of n pi. S xxxi, fig n n f ment ° f length on one side. The septum commences between 



th. 2 2 andth. 2 1 . 



The polypary presents several views when compressed. In bi-profile view 

 the Glimacograptus character of the proximal end is most clearly seen in the 

 obverse aspect, though it is fairly obvious in the reverse also. The excavation 

 is pronounced, and the sigmoid curvature of the ventral walls sharp, though the 

 free edge is often inclined ; towards the distal end in this view the excavation, 

 as such, disappears with the diminution of the curvature of the ventral walls. 



