424 



BRITISH GRAPTOLITES. 



Horizon and Localities. — Lower Wenlock Shales (basal beds). 



Wales : Road between Llanbiynmair and Talerddig ; Tarannon River ? 



Collection. — Wood. 



Figs. 286 a — c. — ilonograptus riccarto- 

 nensis, Lapworth. 



Monograptus riccartonensis, Lapworth. Plate XLII, figs. 8 a—e. 



1876. Monograptus riccartonensis, Lapworth, Geol. Mag. [2], vol. iii, p. 355, pi. xiii, figs. 2 a — e. 



1877. Monograptus riccartonensis, Lapworth, Proc. Belfast Nat. Field Club, pi. v, fig. 23. 



1880. Monograptus riccartonensis, Lapworth, Ann. Mag. Nat. Hist. [5], vol. v, p. 155, pi. iv, fig. 8 c. 

 1883. Monograptus riccartonensis, Tullberg, Skanes Grapt., p. 23, pi. ii, figs. 26, 27. 



Polypary 7 cm. or more in length, straight or with slight dorsal flexure, having 

 an approximately uniform breadth of 1*5 mm.; proximal end slightly 

 reflexed. Sicula conspicuous. Thecae ten to eight in 10 mm., of the general 

 type of M. priodon, but smaller with fully one-half involved in the hook, 

 which, however, occupies only one-quarter of the total breadth. Over- 

 lap one-third to one-half the total length. 



Description. — The polypary has commonly a 

 peculiar limp and " broken-backed " appearance, but 

 some specimens may appear comparatively straight 

 (PI. XLII, fig. 8 e). It widens from an initial 

 breadth of '5 mm. (exclusive of the sicula) to 1*5 

 mm. within the first 3 cm. of length; the breadth 

 is then as a rule maintained for the remainder of 

 its length, though occasionally specimens whose 

 maximum breadth is 2 mm. are met with in some 

 localities. 



The sicula is long, measuring 1*6 mm. in length 

 and reaching nearly to the level of th. 2. 



The thecae throughout call to mind those of the 

 proximal part of M. Sedgwickii when the thecal 

 spine in that species is not well seen. 



Affinities. — Monog. riccartonensis may be regarded 

 as a narrow limp form intermediate in some respects 

 between M. priodon and M. Sedguciclcii. It differs 

 from M. priodon in its narrower limper habit and less conspicuously hooked thecae ; 

 and from M. SedgwicJcii in being narrower, in the absence of spines on the theca?, 

 and in the greater retroversion of the apertural region in the more distal thecse. 



Horizon and Localities. — Wenlock Shales (Riccarton Beds), zone of Gyrtog. 

 Murcliisoni and zone of M. riccartonensis. 



S. Scotland »■ Riccarton; Balmae Shore, Kirkcudbright; Penwhapple Glen; 

 Hindhope ; Edgerston ; Dalton, etc. Wales : Penarth Quarry, Carrog ; Large 



1 





a. Small proximal fragment, showing 



sicnla. On same slab as PI. XLII, 

 fig. 8 b. 



b. Thecse not far from proximal end. 



On same slab as PI. XLII, fig. 8 b. 



c. Distal thecse in relief. Penwhapple 



Glen, Girvan ; Wenlock Beds. Coll. 

 Lapworth . 



