

& 



264 BRITISH GRAPT0L1TES. 



The commonest and most characteristic view is that given in Fig. 180 d, showing 

 apparently that the section of the polypary was somewhat concavo-convex, 

 approximating to that in Amplexograptus. 



Affinities. — Distal fragments of M. modestus bear a certain amount of 



resemblance to Orthoq. vesiculosus and 0. bellulus. 



Fig. 180 (J. — Mesograptus modestus, 



Lapw. It is, however, a smaller form than 0. vesiculosus, 



and has more closely set thecae ; and though resem- 

 bling 0. bellulus in these respects, has a proximal 

 end different in its character. 

 'Jt Horizon and Localities. — Llandovery, Birkhill 



Shales, Skelgill Beds, Dyffryn Flags, etc. (zones of 

 Orthog. vesiculosus and Gephalog. acuminatus). 



S. Scotland : Dobb's Linn ; Frenchland Burn ; 



Belcraig, etc. Wales: Rhayader; Pont Erwyd 



d - D SSLgSt of a p^ e of St lpecSe W n district, Cardiganshire; St. Martin's Cemetery, 



on same slab as pi. xxxi, fig-, ii c . Pembrokeshire. Lake District : Skelgill ; Keisley. 



Ireland : Slate Quarry and Little River, Pomeroy, Co. Tyrone ; Donaghadee, 



Co. Down. 



Associates, etc. — Mesog. modestus is an extremely widespread species, occurring 

 almost everywhere where the Lower Birkhill Shales or their equivalents are known, 

 and being even of commoner occurrence than the zone fossil Orthog. vesiculosus ; in 

 some localities it occurs in such quantities as to constitute a distinct sub-zone. 

 While occurring most abundantly within the limits of the Orthog. vesiculosus zone 

 as originally defined by Lapworth, it occasionally survives into the overlying zone 

 of Monog. gregarius; and somewhat smaller forms, which cannot at present be 

 distinguished from the typical species, occur at the base of and throughout the 

 zone of Geplialog. acuminatus. Its most usual associates at the typical horizon are 

 Orthog. vesiculosus, Glimacog. medius, CI. rectangular is, Monog. tenuis, and various 

 Bhnorphoijrapti. At the underlying horizon it occurs with var. parvulus and 

 Glyptog. persculptus. 



Collections. — Geological Survey of Scotland, British Museum, Sedgwick 

 Museum, Marr, O. T. Jones, H. Lapworth, Lapworth, and the Authors. 



Var. parvulus (II. Lapworth). Plate XXXI, figs. 12 a — d. 



1900. Climacoaraptus parvulus, H. Lapworth, Quart. Journ. Geol. Soc, vol. Ivi, p. 131, fig. 20 a, b. 



The form described by H. Lapworth as Glimacog. parvulus is an extremely 

 diminutive one, not exceeding 4 — 5 mm. in length, and possessing only four to six 

 thecae on either side. The thecas in both aspects of the polypary recall those of 



