ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE OIRVAN DISTRICT. 835 



pointed, very slightly incurved, rising a little above hinge-line ; hinge-area small, 

 triangular, steeply inclined, with triangular delthyrium. Brachial valve (not well 

 known) flatter than pedicle-valve, with smaller inconspicuous not elevated beak and 

 narrower hinge-area. Surface of valve with 12-14 primaries dividing into two or three 

 unequal ribs close to beak, the middle one usually the strongest ; a shorter rib of 

 intermediate size is usually intercalated between the primary fascicles at one-third to 

 one-fourth their length, and near the hinge-line all the ribs are smaller, subequal, and 

 more closely placed ; all the ribs are subangular and straight. 



Dimensions. — Length, 13 mm. ; width, 19 mm. 



Horizon. — Stinchar Limestone Group. 



Locality. — Aldons. 



Remarks. — The two specimens on which the above description is based were 

 labelled 0. intercostata, Portl.,* by Davidson, and onef of his figured specimens of 

 this species is stated in the explanation of the plate to be from Girvan, but he 

 does not give this locality in the description on p. 236, though in his Supplement 

 it is entered in the list on p. 226 as occurring at Craighead, Aldons, and in the 

 Balclatchie conglomerate. However, these specimens from Aldons do not agree in 

 their ribbing with the type of 0. intercostata, Portlock, from Tyrone (Davidson, op. 

 cit., iii, pi. xxxviii, figs. 1, 3), and they more resemble 0. Jissicosta, Hall,j particularly 

 in respect of the ribs, though the latter divide closer to the beak in our species than 

 in the American one. 



Orthis (Plectorthis) subplicatella, sp. nov. 

 (Plate VI, figs. 22-26.) 

 Shell transversely subelliptical ; unequally biconvex ; hinge-line rather less than 

 maximum width of shell ; cardinal angles obtuse. Pedicle-valve strongly convex, 

 somewhat flattened anteriorly in middle ; beak rounded, incurved ; hinge-area 

 moderate, triangular, nearly in plane of valve. Brachial valve much less convex 

 than opposite valve, somewhat flattened ; beak small, inconspicuous, not incurved ; 

 hinge-area smaller than that of opposite valve, steeply inclined or at right angles to 

 plane of valve. Surface of valves covered with about 30 straight simple regular 

 equal quadrate or rounded ribs with vertical sides, equidistant, rather wider than the 

 interspaces, which are deep, regular, rounded ; both covered with close regular con- 

 centric striations. Interior of pedicle-valve with small teeth, and with oblong to 

 subpentagonal muscle-scar, truncate anteriorly, about one-third (or less) the length 

 of valve, deeply sunk, with sharp lateral marginal edges and long subparallel sides 

 slightly indented at about half length ; diductors duplicate on each side, enclosing- 

 narrow adductor scar. 



* Portlock, Geol. Rep. Londond., p. 454, pi. xxxvii, fig. 3. 

 t Davidson, op. cit., iii, p. 236, pi. xxxviii, fig. 4. 



I Hall, Palxont. N.Y., vol. i, 1847, p. 121, pi. xxxii, fig. 7; Foerste, Bull. Denison Univ., vol. xiv (190D), 

 p. 221, pi. iv, figs. 5a, b; ibid., vol. xvi (1910), p. 48, pi. vi, fig. 4. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 121 



