ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE CxIRVAN DISTRICT. 837 



nearly all primaries (especially in young individuals), and increasing in number by 

 regular intercalation close to beak (and very rarely again), separated by equal 

 rounded or subquadrate interspaces ; all ribs equal in size and equidistant on margin 

 and numbering 50-GO in the larger shells. Ribs and interspaces crossed by strong 

 close concentric striations. Ribs show strongly on interior of shell. 

 Dimensions. — 



(1) 



(2) 



(3) 



15 



13'5 



14 '5 mm. 



20 • 



18'0 



20"0 mm. 



Length . 

 Width . 



Horizon. — (l) Drummuck Group (Starfish Bed) ; (2) Whitehouse Group? 



Locality.— (l) Thraive Glen ; (2) Shalloch Mill. 



Remarks. — This shell may be placed in the subgenus Plectorthis. In the shape, 

 regularity, and simplicity of the ribbing it recalls many shells referred to the species 

 0. calligramma by different authors, and may also be compared to 0. duftonensis* 

 The interior of the brachial valve specially resembles that of O. plicata, Sow.t 

 Fragmentary specimens are difficult to distinguish from O. porcata, M'Coy. Some 

 poorly preserved shells from Shalloch Mill, of which the internal characters are 

 unknown, were labelled by Davidson O. soiverhyana and 0. calligramma, but agree 

 in shape and ribbing with the Drummuck examples of 0. thraivensis. 



Subgenus dinorthis, Hall and Clarke. 

 Orthis (Dinorthis) fiabellulum, Sowerby, var. nov. carrickensis. 

 (Plate VII, figs. 2-6.) 



1883. Orthis flabellulum, Sowerby, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., 

 p. 179, pi. xiii, figs. 3-5 (non fig. 6). 



Shell piano- to concavo-convex, semicircular to semielliptical ; length to width 

 about 4:5; hinge-line less than width of shell ; cardinal angles rounded or obtuse. 

 Pedicle-valve subconical, flattened or slightly concave, shallow, deepest at beak, re- 

 supinate ; beak small, not incurved nor much elevated, pointed ; hinge-area at right 

 angles to plane of valve, moderately large, triangular. Brachial valve gently con- 

 vex ; beak small, inconspicuous ; hinge-area smaller than that of opposite valve and 

 lying in plane of valve. Surface of valves covered with straight radiating rounded 

 ribs, of equal size, increasing by fairly regular bifurcation at about one-third (or 

 less) their length, numbering 50-60 on margin ; interspaces equal to or narrower 

 than ribs, rounded ; ribs and interspaces crossed by fine concentric striae. Interior 

 of pedicle-valve with short subquadrate well-defined muscle-scar, about one-fourth 

 (or less) the length of valve, weakly emarginate behind. 



Remarks. — The large resupinate shells from Craighead which Davidson referred 



* Reed, Geol. Mag., dec. v, vol. vii, 1910, p. 295, pi. xxiv, figs. 5-11. 



f Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 245, pi. xxxv, figs. 25, 26 ; pi. xxxvii, fig. 1. 



