ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 907 



interspaces holding 2-3 smaller and rather shorter ribs, of which one usually becomes 

 larger than the others towards the margin ; ribs and interspaces crossed by strong- 

 close concentric lineation. 



Dimensions. — Length, 8'0 mm. ; width, 12 "5 mm. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Remarks. — This small shell seems to belong to the same genus as Sch. applanata 

 and Sch. pecten, the former of which it much resembles in its ribbing and general 

 aspect. The cardinal process and crura, so far as they are preserved, appear to 

 possess the typical structure of this genus, and the whole character of the shell 

 suggests its reference to Schuchertella rather than to Strophomena, though its 

 occurrence on an Ordovician horizon is unexpected. 



Schuchertella pertinax, sp. no v. 



(Plate XIX, fig. 23 ; Plate XX, figs. 1-3.) 



1883. Strophomena pecten (Linne), Davidson (pars), Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., 

 p. 194, pi. xv, fig. 15 (non Str. pecten, Davidson, ibid., vol. iii f p. 304, pi. xliii, figs. 1-11). 



Shell semielliptical to subquadrate, large, flattened, compressed ; hinge-line equal 

 to or rather less than maximum width ; cardinal angles rectangular or slightly 

 obtuse. Pedicle-valve flatter than brachial valve or very slightly concave, except 

 near beak, where it is gently convex ; beak small, pointed, not projecting above 

 hinge-line ; hinge-area narrow, steeply inclined, longitudinally striated, with small 

 triangular convex deltidium and inner " secondary area." * Interior of pedicle-valve 

 with large short stout triangular teeth, and pair of large indistinct flabelliform 

 diductors about one-third the length of the shell ; genital areas coarsely and radially 

 pustulate. Brachial valve very gently convex, flattened ; beak small, inconspicuous ; 

 hinge-area linear, narrower than in opposite valve. Interior of brachial valve, with 

 cardinal process low, sessile, broad, stout, fused with crural plates, arising at right 

 angles to floor of valve, divided into a pair of low rounded subparallel oval lobes 

 closely placed, highest in front, facing posteriorly, grooved (?) ; crural plates short, 

 strong, slightly curved, making angle of 30°-40° with hinge-line ; low weak broad 

 median ridge running forward from base of cardinal process between very faintly 

 impressed adductor scars. 



Surface of shell ornamented with about 30-40 primary riblets, regular, rounded, 

 or subangular, equidistant, equal, and straight, except those on lateral portions near 

 hinge-line, which are very gently curved back ; interspaces wide, flat, holding 2-6 

 much finer thread-like radii increasing in number by intercalation, and 1 or more of 

 them becoming stronger at one-third to one-half their length, and as strong on 

 margin as primaries, so that 70-90 strong equal equidistant riblets are found around 



* Thomas, "Brit. Carb. Orthotetinae " (Mem. Geol. Surv., vol. i, pt. ii, 1910), p. 97, fig. 2. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 130 



