896 DR F. R. C. REED ON THE 



Subgenus brachyprion, Shaler. 



Stropheodonta {Brachyprion) columbaria, sp. nov. 



(Plate XVII, tigs. 13-19.) 



1883. Orthis sagittifera, M'Coy, Davidson, Mon. Brit. Fuss. Brack., vol. v, Si/ur. Suppl., p. 181, 



pi. xiii, fig. 29 (non Orthis sagittifera, M'Coy, 1851). 

 1883. Strophomena expansa, Sowerby, Davidson (pars), ibid., pp. 194, 227. 



Shell subquadrate, nearly as long as wide, unequally biconvex, widest at hinge- 

 line ; cardinal angles rectangular or slightly less than right angles, rarely alate. 

 Hinge-line furnished with 6-10 small oblique transverse crenulations for a short 

 distance on each side of delthyrium. Pedicle-valve gently convex, most so along 

 median line, often subcarinate, deepest just behind middle, more or less flattened 

 towards cardinal angles; beak small, acute, rising well above hinge-line; hinge-area 

 triangular, moderately high, steeply inclined, with wide triangular delthyrium, 

 frequently narrowed by lateral ingrowths of margins. Interior of pedicle-valve with 

 pair of stout teeth inclined at 60°-70° to hinge-line ; muscle-scar deeply impressed 

 behind, but weak and indistinctly defined in front, composed of pair of rather large 

 fiabelliform diductors enclosing small oval raised adductor scar ; ovarian areas pitted. 

 indefinite. Brachial valve more uniformly convex than opposite valve, occasionally 

 with a weak median depression ; beak small, inconspicuous ; hinge-area narrower 

 than that of pedicle-valve, nearly at right angles to plane of valve, with triangular 

 delthyrium. Interior of brachial valve with prominent thick anterior walls to dental 

 sockets, and stout cardinal process composed of pair of closely placed triangular lobes 

 deeply grooved on posterior face ; hinge-plate thick, produced anteriorly into low 

 broad rounded median ridge dying out anteriorly, and with small median tubercle 

 situated immediately in front of cardinal process ; adductor muscle-scars weakly im- 

 pressed, indistinct, apparently consisting of two pairs ; ovarian areas strongly pitted, 

 indefinite. Surface of valves covered with numerous regular closely placed radiating 

 small rounded or subangular ribs, slightly curved back on each side, with shorter 

 rather finer ones intercalated rather irregularly between the primaries, usually 

 at one-third and again at one-half to three-fourths their length, and with very fine 

 concentric striation over all. 



Dimensions. — Length, 18-24 mm; width, 20-27 mm. 



Horizon. — Mulloch Hill Group. 



Localities. — Mulloch Hill, Craigens, East Threave Farm. 



Remarks. — The crenulations of the inner part of the hinge-line are rarely visible 

 owing to the state of preservation of the specimens. But the bilobed cardinal 

 process at once separates it from the true Orthis sagittifera, M'Coy, # to which 

 Davidson referred some examples, and the coarse ornamentation as well as the internal 

 features render it difficult to see how Davidson was led to label some other specimens 

 "Strophomena" expansa, Sowerby. The reference to the subgenus Brachyprion 



* M'Coy, Syn. Brit. Palmoz. Foss. Woodw. Mus., 1852, p. 227, pi. iH, figs. 15-19. 



