842 DR F. R. C. REED ON THE 



but elsewhere on margin nearly equal in size and numbering in all 100-130. Coarse 

 concentric close striation over ribs and interspaces. Ribs apparent near margins in 

 interior of shell. 



Dimensions. — Average length, 24 mm. ; width, 30 mm. ; thickness, 14 mm. 



Horizons. — (l) Balclatchie Group (including conglomerate) ; (2) Stinchar Lime- 

 stone Group ? 



Localities. — (l) Balclatchie, Dow Hill ; (2) Doularg. 



Remarks. — The best-preserved specimens come from Balclatchie ; those from the 

 conglomerate, which Davidson labelled O. soiverbyana (op. cit., Swppl., p. 178), have 

 rather finer ribs, but there is in each case some variation in their strength. This 

 shell must undoubtedly be put in the subgenus Hebertella, and is allied to 

 0. alveata-richmondensis, Foerste, # 0. subjugata, Hall,t O. occidentalism Hall,| 

 and 0. sinuata, Hall.§ One specimen from Doularg (fig. 31) labelled by Davidson 

 as a "most characteristic specimen of 0. conjinis" seems to be referable to our 

 new species. 



Orthis (Hebertella) crispa, M'Coy. 



(Plate VIII, figs. 3-7.) 



1846. Orthis crispa, M'Coy, Si/n. Silur. Foss. Ireland, p. 29, pi. iii, fig. 10. 



1870. Orthis crispa, M'Coy, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 256, pi. xxxviii, tigs. 5-10. 



1883. Orthis crispa, M'Coy, ibid., vol. v, Silur. Suppl., p. 176, pi. xiii, fig. 7 (non 8). 



Shell unequally biconvex, subquadrate to semicircular or transversely semi- 

 elliptical, from 1^ to l|- times as wide as long ; hinge-line usually rather less than 

 width of shell or just equal to it ; cardinal angles obtuse or slightly rounded. 

 Pedicle-valve moderately convex, somewhat flattened in middle near anterior margin, 

 or weakly depressed ; beak rather high, pointed; not much incurved or swollen ; 

 hinge-area moderate, steeply inclined. 



Brachial valve strongly convex, more swollen than pedicle-valve, and usually 

 without any median depression ; beak much lower and smaller (but more incurved) 

 and hinge-area much narrower than in opposite valve. 



Interior of pedicle-valve with completely circumscribed sunken muscle-scar, 

 rounded to subtruncate in front, one-fourth to one-third the length of valve, with 

 broad adductor impression between the pair of diductors ; teeth small. 



Interior of brachial valve with transverse thickened cardinal plate prolonged in 

 front into short ridge between adductor scars ; cardinal process pear-shaped, widest 

 at base; crural plates stout, triangular, prominent; posterior adductors well marked, 

 sunken ; anterior adductors faint. 



Ribs angular, closely placed, subequal, increasing by bifurcation once or twice or 

 occasionally by trifurcation and intercalation ; lateral ribs curve back on each side ; 



* Foerste, Bull. Denison Univ., vol. xvi (1910), art. 2, p. 55, pi. v, figs. 10, 17. 



t Foerste, op. cit., vol. xvii (1912), p. 129, pi. viii, fig. 6. 



\ Foerste, op. cit., vol. xvi, p. 53, pi. ii, figs. 1, 2. § Foerste, op. cit., vol. xvi, p. 52, pi. ii, tig. 5. 



