(1) 



(2) 



5 '5 mm. 



4'3 mm. 



10'6 mm. 



8 '2 mm. 



ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 885 



about 60°-75°, extending about one-fifth to one-fourth the length of the valve, with 

 swollen boss between them ; vascular trunks diverging at about 60°, strong, well 

 marked, deeply sunk, running along diductors, and extending beyond them for about 

 half the length of valve, rarely provided with lateral branches. Brachial valve flat 

 or very slightly concave ; cardinal process tripartite ; interior with large triangular 

 muscle-scar formed by pair of fiabelliform. contiguous double muscle-scars raised on 

 thickened platform, extending fully half the length of valve and radially ridged. 

 Interiors of both valves covered with radially arranged small pustules becoming 

 larger towards margins. External surface of shell covered with 20-28 strong 

 radii, mostly developed only near margin, with 4 or 5 smaller thread-like lines 

 between them ; lateral portions of valves marked with 9-10 more or less wavy 

 concentric wrinkles meeting hinge-line at 45°-60°. 

 Dimensions. — 



Length . 



Width .... 



Horizon. — Balclatchie Group. 



Localities. — Ardmillan Brae, Dow Hill, Balclatchie. 



Remarks. — This shell is distinguished from the other varieties by the concentric 

 rugse on the surface and the denticulation of the cardinal margin. The internal 

 and muscular characters are much like those of a specimen of PI. sericea from 

 Haverfordw r est, figured by Davidson (op. cit., iii, pi. xlviii, fig. 19). The concentric 

 wrinkling and general external appearance resemble PI. sericea, var. aspera 

 (James),* from the Trenton Formation of New York, but in the latter no mention 

 is made of the denticulation of the hinge-iine. 



Plectambonites sericea (Sowerby), var. nov. thraivensis. 

 (Plate XV, figs. 27-32.) 

 Shell semielliptical to semicircular, with acute slightly produced cardinal angles, 

 rarely semicylindrical. Pedicle-valve moderately convex, most so in middle, more 

 or less inflated, with lateral portions somewhat depressed or flattened ; beak small, 

 slightly incurved, not swollen nor overhanging ; hinge-area rather large, steeply 

 inclined or in plane of valve. Interior of pedicle-valve with pair of very small 

 indistinct diductors and pair of short strong straight subparallel vascular trunks 

 dividing at about one-fourth to one-third the length of valve into 2 or 3 smaller sub- 

 parallel branches. Brachial valve gently concave ; interior with pair of large broad 

 fiabelliform very weakly impressed muscle-scars diverging at about 20°, with sharp 

 straight raised inner margins, less than half the length of valve. Whole interior of 

 both valves rather coarsely pustulate. Surface of shell marked with 12-20 primary 



* James, Cincinnati Quart. Journ. Sc, 1874, i, p. 151 ; Meek, Palxont. Ohio, 1873, i, p. 70, pi. v, fig. 3 ; 

 Rcedemann, Bull. 49, New York State Mus., 1901, p. 18, pi. i, figs. 6, 7. 



