860 DR F. R. C. REED ON THE 



this species in dealing with fairly well-preserved exteriors, as the contour of the 

 valves and shape of the shells as well as the ribbing are different, and internally the 

 muscle-scars and hinge-apparatus easily mark them oft". 0. mullochensis is quite 

 distinct specifically from 0. reversa, of which Davidson * at first regarded it as a 

 variety, though subsequently t he stated that it might be distinct. I have not seen 

 any examples from the Upper Bala of Thraive Glen, in which Davidson says it 

 occurs ; and it seems strictly confined to the Lower Llandovery. 



Horizon. — Mulloch Hill Group. 



Localities. — Mulloch Hill, Craigens, Auld Thorns. 



Subgenus nicolella, nov. 



Definition. — -Shell transverse, semicircular, more or less alate, plano-convex to 

 concavo-convex, widest along hinge-line. Brachial valve flat or slightly concave. 

 Pedicle-valve strongly convex, more or less inflated, with large obtuse swollen 

 incurved beak, large hinge-area, and somewhat flattened cardinal angles. Surface 

 of valves covered with a few coarse angular or subangular prominent ribs, usually 

 simple, straight and separated by equal or subequal angular interspaces. Interior of 

 pedicle- valve with small teeth, and broad transverse subcircular weakly impressed 

 muscle-scar. Interior of brachial valve with large prominent lanceolate or oval 

 cardinal process and small very widely divergent crura. Type : Orthis Actonise, 

 Sowerby. * 



It seems impossible to place this shell naturally in any of the previously recog- 

 nised subgenera of the genus, the combination of characters differentiating it from 

 all of them. Davidson § himself thought its features so peculiar that he regarded 

 it as simulating certain forms of " Strophomena." The forking of the ribs, on which 

 Sowerby laid much stress, is not general, the majority of the Welsh specimens 

 having simple undivided ribs. The type specimens came from Acton Scott, near 

 Church Stretton. The subgenus is named after James Nicol. 



Orthis {Nicolella) Actonise, Sowerby. 



(Plate X, figs. 25-27.) 



1839. Orthis Actonim, Sowerby, in Murchisou's Silurian System, p. 639, pi. xx, fig. 16. 

 1868. Orthis Adoince, Sowerby, Davidson (pars), Mon. Brit. Foss. Brach., vol. iii, pt. viii, p. 252, 

 pi. xxxvi, figs. 5-12, 14-17 (non 13). 



The typical form of this well-known species occurs rarely in the Girvan area and 



only with certainty at Shalloch Mill. Specimens from this locality agree closely 



with some of the examples from other places figured by Salter || and Davidson. ^| 



The small number, regularity, sharpness, prominence, and simplicity of the ribs and 



* Davidson, op. cit., vol. iii, p. 221, pi. xxix, figs. 11-13. f Ibid., vol. v, p. 180, pi. xvi, figs. 14, 15. 



I Sowerby in Murchison's Silur. Syst., p. 639, pi. xx, fig. 16. $ Davidson, op. cit., vol. iii, p. 252. 



|| Salter, Mem. Geol. Surv., vol. iii, p. 339, pi. xxi, figs. 1-8. 

 IT Davidson, op. cit., vol iii, pi. xxxvi, figs. 8-10. 



