848 DR F. R, C. REED ON THE 



are typically provided with short, pointed ears, the cardinal angles being produced, 

 and this form I would regard as characteristic since it comes from the type locality. 



The subgeneric position of this shell is probably in Heterorthis, Hall and Clarke,* as 

 it seems allied to 0. Clyde, Hall,t and to its relations ; but it may belong to the group 

 containing 0. deflecta, Conr.,j or to the new subgenus Harhnessella (see p. 862), for 

 the external characters of specimens from the type locality (Plate VIII, figs. 26, 26a) 

 recall 0. vespertilio. 



Horizon.— Stmch&v Limestone Group. 



Localities. — Bogang, Minuntion, Doularg, Benan Burn, Tramitchell, Craighead. 



Subgenus bilobites, Linne. 

 Orthis (Bilobites) biloba (Linne). 

 (Plate IX, figs. 1, 2.) 

 1767. Anomia biloba, Linne\ Syst. Wat., ed. xii, vol. i, p. 1154. 

 1868. Orthis biloba, Linne, Davidson, Mon. Brit Foss. Brack., vol. iii, pt. vii, p. 206, pi. xxvi, figs. 10-15. 



This species occurs in the Middle Llandovery of Woodland Point and appears to 

 agree closely with the usual Wenlock Limestone form, but it differs in some respects 

 from the Ordovician form found in the Girvan area which is described below as a 

 distinct variety. The rib along the crest of each lobe of the shell in the Woodland 

 Point shell is straight and sometimes rather stronger than the others, and it has 

 usually 1 or 2 short ribs two-thirds or three-fourths its length intercalated on each 

 side between it and the lateral ones ; the latter diverge on each side and number 

 10-12 on each lobe ; there is no marked central rib in the sinus of the pedicle-valve 

 as in the Whitehouse Bay form, and the ribs are coarser, stronger, and more equal in 

 size. Davidson (op. cit., Suppl., p. 180) only records this species in Scotland from the 

 Ludlow of the Pentland Hills and Shalloch Mill and says it is very rare. 



Horizons. — (l) Saugh Hill Group ; (2) Camregan Group. 



Localities. — (l) Woodland Point ; (2) Bargany Pond Burn. 



Orthis (Bilobites) biloba var. 

 (Plate IX, figs. 3-6.) 



1883. Orthis biloba, Linne, Davidson, Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., pp. 180, 225. 



In this variety one valve, the pedicle-valve, is strongly convex, the other is con- 

 cave and resupinate. The lobes of the pedicle-valve are subcarinate, and each crest 

 is usually marked by a rib stronger than those on the slopes ; the median sinus has 

 also a decidedly strong simple central rib with weaker finer ribs on each side of it in 

 the floor of the sinus ; 5-7 ribs, increasing in size and becoming more distant, ornament 

 the inner slopes of the lobes, and on the outer side of the lobes similar ribs are present, 

 closing up and becoming smaller near the cardinal angles ; all the ribs are simple and 



* Hall and Clarke, Palseont. New York, vol. viii, 1892, Brach., i, p. 202. f Ibid , pi. vb, figs. 20-24. 



1 Ibid., pi. va, figs. 28-30; Winchell and Schuchert, Palseont. Minnes. GeoL Surv., vol. iii, 1893, p 422, 

 pi. xxxii, figs. 24-30. 



