834 DR F. R. C. REED ON THE 



examples. The ribs are subangular, and there are about 20-30 primaries with shorter 

 and smaller ones intercalated rather irregularly as regards number and length, but 

 usually at about one-third and again at about three-fourths ; rarely do the primaries 

 divide. The interspaces between them are somewhat unequal and irregular in size. 

 The lateral ribs curve back slightly. On the margin the ribs are unequal in strength. 

 All are crossed by close concentric lamellations. In no case are they as numerous 

 as in the typical 0. rustica from the Wenlock Limestone. The internal characters 

 of this variety were not depicted by Davidson, but show some noticeable features. 

 In the pedicle-valve the muscle-scar forms an elongated pentagon, nearly twice as 

 long as wide, and is sunken and well denned ; it is rather less than one-third the 

 length of the shell and is truncated in front ; the diductors are distinct, and separated 

 by a narrow median ridge. In the brachial valve the cardinal process is narrow, 

 short, and linear, and the crural plates are short and widely divergent ; the two pairs 

 of adductors are usually feebly impressed. Davidson* was incorrect in saying the 

 muscle-scar in the pedicle-valve was saucer-shaped if he meant this description to 

 apply to var. rigida. It may be necessary to regard this variety as a distinct 

 species on account of this difference, which sharply distinguishes it from the typical 

 0. rustica. 



Horizon. — Saugh Hill Group. 



Localities. — Woodland Point, ? Shalloch Forge. 



Orthis (Plectorthis) rustica, var. ivalsalliensis, Davidson. 

 (Plate VI, fig. 20.) 



1848. Orthis Walsallii, Davidson. Bull. Soc. Geol. France, ser. 2, vol. v, 1848, p. 322, pi. iv, fig. 7. 

 1868. Orthis rustica, var. wallsalliensis, Davidson, Mon. Brit. Foss. Bra -A., vol. iii, pt. vii, p. 238, 

 pi. xxxiv, figs. 20-22. 

 ?1883. Orthis rustica, var. walsalliensis, Davidson, ibid., vol. v, Sifur. SuppL, p. 226. 



Davidson described this variety originally from the Wenlock Limestone of 

 Walsall, but subsequently figured examples from Eushall Canal and Build was, and 

 mentioned it as occurring in the Girvan area at Penkill. Some poor specimens 

 from Penwhapple Glen appear to be referable to it, judging from their shape ami 

 numerous ribs. 



Horizon. — Penkill Group. 



Locality. — Penwhapple Glen. 



Orthis (Plectorthis) subjissicosta, sp. nov. 

 (Plate VI, fig. 21.) 



1883. Orthis iutercostata, Portlock, Davidson, Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 220. 

 Non 1843. Orthis intercostata, Portlock, Geol. Rep. Londond., p. 454, pi. xxxvii, fig. 3. 



Shell transversely semielliptical ; hinge-line equal to width of shell ; cardinal 



angles rectangular. Pedicle-valve gently convex, most so near beak ; beak small, 



* Davidson, op. cit., vol. iii, p. 238, pi. xxxiv, figs. 18, 19. 



