836 DR F. R. C. REED ON THE 



Interior of brachial valve with sublanceolate cardinal process, stout triangular 

 crural plates ; adductor scars widely separated, posterior pair subcircular, more 

 distinct, and larger than anterior pair. Ribs show internally round margins of 

 shell. 



Dimensions. — Length, 10-11 mm. ; width, 15-16 mm. 



Horizon. — Balclatchie Group. 



Localities. — Balclatchie, Ardmillan. 



Remarks. — This shell agrees in shape and general characters of ribbing and in- 

 ternal characters with O. plicatella, Hall,* of the Hudson River and Trenton Groups. 

 The shape of the muscle-scar in the pedicle-valve recalls O. Whitfieldi, Winched, t 

 Our species is allied to 0. calligramma and 0. ardmillanensis in the regularity, 

 shape, and simplicity of its ribs, and likewise to 0. duftonensis, but it cannot well 

 be associated with any of these species in any strict conception of their specific 

 limitations on account of the nature of the muscle-scars. 



The general characters of the exterior and interior suggest the group Austinella,\ 

 in which Foerste § places 0. Whitfieldi, and of which 0. scovillei, Miller, is the geno- 

 type. || 



Orthis (Plectorthis) thraivensis, sp. nov. 

 (Plate VI, figs. 27-32 ; Plate VII, fig. 1.) 



Shell transversely semielliptical to subquadrate, plano-convex, or slightly concavo- 

 convex ; cardinal angles rectangular ; hinge-line equal to maximum width of shell. 



Pedicle-valve forming a low cone, flattened or slightly concave, less deep than 

 opposite valvje except at beak. Beak forming apex of .cone, pointed, not incurved, 

 higher than that of brachial valve. Hinge-area large, triangular, steeply inclined or 

 nearly at right angles to plane of valve ; delthyrium large, triangular. 



Brachial valve gently convex ; beak small, not prominent, slightly incurved ; 

 hinge-area smaller than that of pedicle-valve, vertical or nearly so. 



Interior of pedicle-valve with subcircular to subpentagonal well-circumscribed 

 muscle-scar, less than one-third length of valve, composed of two simple diductors 

 obscurely separated by narrow linear adductors, with sharp curved marginal ridges 

 defining it at sides but weaker in front. 



Interior of brachial valve with narrow thin compressed linear cardinal process, 

 stout cardinal plate produced into low broad median ridge separating obscurely 

 marked adductor scars ; dental sockets with thick anterior walls. 



Surface of walls covered with narrow rounded regular straight ribs of equal size, 



* Hall, Palxont. New York, vol. i, 1847, p. 122, pi. xxxii, fig. 9 ; Meek, Palseont. Ohio, vol. i, 1873, p. 108, 

 pi viii, fig. 7: Hall and Clarke, op. cit., pi. v, figs. 18-20. 



t Winchell and Schuchert, Palseont. Minnesota, vol. iii, pi. xxxiii, fig. 8, p. 437. 



I Fokrkte, Bull. Deyiison Univ., vol. xiv, 1909, p. 225. 



S Foerste, ibid., vol. xvii, 1912, p. 131. || Foerste, ibid., p. 130, pi. viii, figs. 8a, b, c. 



