ORDOVIOIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 859 



pedicle-valve and have not any divergence in front ; (4) the hinge-line is relatively 

 longer; (5) the cardinal angles are less rounded; (6) the hinge-area is larger; and 

 (7) the fold on the brachial valve is more independent in its elevation and more 

 defined, particularly near the margin. There is a danger of confusing it with Triplecia 

 spiriferoides from the exterior, but the internal characters are quite distinct. 



A large number of well-preserved internal casts allow us to revise the exist- 

 ing descriptions of 0. fallax by the following details : — Shell transversely oval, 

 anterior margin sinuated ; hinge-line straight, less than width of shell ; cardinal 

 angles obtuse. Brachial valve convex, deeper than pedicle-valve, with low median 

 rounded fold arising at about one-third the length of the valve and increasing in 

 height and width anteriorly but not sharply marked off from lateral lobes ; beak 

 small, inconspicuous, rising above hinge-line, incurved with triangular vertical hinge- 

 area in plane of valve. Pedicle-valve shallower than brachial valve, with broad 

 shallow rounded indefinite sinus widening anteriorly and starting at about half the 

 length of the shell ; beak high, more prominent than that of opposite valve, very 

 slightly incurved, with larger triangular hinge-area below it steeply inclined to plane 

 of valve. 



Interior of brachial valve with prominent oval or lanceolate cardinal process, and 

 small dental sockets with much thickened anterior walls bearing stout curved crura ; 

 low tapering median ridge, broad at base, extending forwards from thick cardinal 

 plate between weakly marked adductors ; ovarian, markings on each side distinct. 



Interior of pedicle-valve with teeth forming thin short dental plates ; muscle-scar 

 oval, more than one-third length of valve, sharply enclosed at sides by curved 

 elevated ridges nearly meeting in front ; diductors scars elongate, enclosing narrow 

 lanceolate pair of adductors. Ovarian areas large, coarsely pitted, and marked with 

 irregular radial grooves. Surface of shell ornamented with closely placed very fine 

 regular straight rounded or subangular riblets of equal or subequal size, increasing 

 by bifurcation and interpolation at about half their length, so as to number about 

 100-120 on the margin ; ribs slightly curved back on lateral slopes and becoming 

 smaller nearer hinge-line. 



Dimensions. — Average length, 15-16 mm. ; average width, 19-20 mm. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Orthis (Schizophorella) mullochensis, Davidson (emend.). 



1868. Orthis reversa, Salter, var. mullochiensis, Davidson, Mon. Brit. Foss. Brack., vol. iii, p. 221, 



pi. xxix, figs. 11-13. 

 1883. Orthis mullochiensis, Davidson (pars), ibid., vol. v, Silur. Suppl., p. 180. 



The full description and figures of this species sufficiently define its characters, 

 but many of the shells which Davidson labelled as O. mullochensis in Mrs Gray's 

 collection belong to O. crassa or other species. There is, however, no need to confuse 



TKANS. R07. SOC. EDIN., VOL. LI, PART IV (NO. 26). 124 



