ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 831 



from the Balclatchie Beds, which mostly occur as internal casts with external 

 impressions ; and it is possible that these should be separated as a variety. 



The Shalloon Mill examples more closely resemble the shells from Craighead 

 than those from Balclatchie, and are especially comparable to the above-mentioned 

 0. lyckholmiensis, Wysog. ; the longitudinal striations of the Craighead and 

 Balclatchie forms seem, however, missing. This species is dedicated to the 

 distinguished Scotch professor, John Playfair. 



Subgenus plectorthis, Hall and Clarke. 



Orthis (Plectorthis) ardmillanensis, sp. nov. 



(Plate V, figs. 33-39.) 



Shell semicircular to semielliptical ; hinge-line equal to width of shell ; cardinal 

 angles rectangular or slightly pointed. Pedicle-valve subcorneal, slightly convex ; 

 beak pointed, not incurved ; hinge-area large, at right angles to plane of valve or 

 steeply inclined, with triangular delthyrium. Brachial valve flattened or slightly 

 concave except near beak, with a broad very shallow median depression widening to 

 margin ; beak small, inconspicuous ; hinge-area steeply inclined, smaller than that on 

 pedicle-valve. Interior of pedicle-valve with small broad subcircular or transverse 

 well-defined muscle-scar about one-fourth (or less) the length of valve ; teeth small, 

 supported on dental plates. Brachial valve with thickened hinge-plate, thin laminar 

 cardinal process, and large stout crura. Posterior adductors well marked, oval, widely 

 separated ; anterior adductors smaller, weaker. Surface of valves ornamented with 

 28-35 simple rounded or quadrate equal and equidistant ribs, separated by rounded 

 interspaces of equal size. Ribs and interspaces crossed by delicate concentric 

 striae, and usually marked with a few fine longitudinal strise, of which the one in 

 the middle of each interspace is the most prominent. Ribs show near margins of 

 interior of shell. 



Dimensions. — Length (average), 10 mm. ; width (average), 12 mm. 



Horizon. — Balclatchie Group. 



Localities. — Ardmillan, Balclatchie (conglomerate) ? 



Remarks. — In its essential characters this new species much resembles O. trice- 

 naria, Conrad, # and Davidson t himself described a shell from Piedmont Glen, 

 apparently belonging to our species, as O. tricenaria with a query. In its ribbing it 

 much resembles 0. calligramma, but in its general shape and in the contour of the 

 valves it is different. The ribbing is not quite the same as in 0. tricenaria, and is 

 more like that in the Girvan species described below as 0. subplicatella, but the 

 nature of the muscle-scar in the pedicle-valve and the conical shape of that valve 

 distinguish it. O.-duftonensis, Reed, J differs by the greater convexity of the brachial 



* Winchell and Schuchert, Faleeont. Minnesota, vol. iii, p. 418, pi. xxxii, figs. 18-20. 



t Davidson, op. cit., iii, p. 276, pi. xxxviii, fig. 28. 



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



