876 DR F. R. 0. REED ON THE 



not produced. The beak also is somewhat swollen and overhanging. The interior of 

 this valve is incompletely known, but in one cast from Craighead (fig. 31) the diductors 

 appear to be small, short, and triangular, and there is a pair of median subparallel 

 vascular trunks which border laterally a coarsely pitted median oblong space about 

 two-thirds the length of the valve, bending in at its anterior end and giving off here a 

 small outer divergent branch, but themselves breaking up as they run forwards into 

 several smaller branches. In some Ardmillan specimens probably referable to this 

 species there are several lateral vascular trunks diverging from the diductors. 



The ornamentation of the exterior is very faint, as Davidson remarked, but it 

 seems to consist of very delicate radial striae without primary radii. 



This species appears to be closely allied to Plectamb. pisum, Ruedemann,* from 

 the Trenton conglomerate of Rysedorph Hill, near Albany, N.Y. ; for its shape, relative 

 convexity, and internal characters bear a considerable resemblance. Rttedemann com- 

 pares his species with PL scissa (Salter), but this is undoubtedly distinct (see p. 880), 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group. 



Localities. — (l) Craighead; (2) Balclatchie (conglom.), Ardmillan ? 



Plectambonites Uandeiloensis (Davidson). 

 (Plate XIII, figs. 32-34 ; Plate XIV, figs. 1-3.) 



1883. Leptxna Uandeiloensis, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 171, 

 pi. xii, figs. 26, 26a, b (non figs. 27-29). 



Shell transversely semielliptical, strongly concavo-convex, widest along hinge- 

 line ; cardinal angles rectangular ; hinge-line minutely crenulated. Pedicle-valve 

 strongly and uniformly convex, swollen, rounded ; beak small, pointed, slightly in- 

 curved, scarcely projecting ; hinge-area narrow, in plane of valve. Brachial valve 

 strongly concave, with more or less flattened disc ; hinge-area narrow, nearly at right 

 angles to disc. Interior of pedicle-valve with low rounded concentric thickening of 

 shell close to margin ; teeth small, stout, triangular, widely separated ; diductor 

 muscles deeply sunk, well defined, short, triangular, strongly divergent, weakly 

 bilobed by shallow notch on anterior margins, posteriorly embracing well-marked 

 oval adductor scar ; pair of vascular trunks running forwards with slight divergence 

 for about half length of valve from anterior inner angles of diductors, each trunk then 

 dividing into a short inner and a long outer branch widely diverging and giving off 

 short subparallel branches to margin. Interior of brachial valve with small bilobed (?) 

 cardinal process, and thickened undefined cardinal plate giving off laterally a long 

 curved somewhat undulating ridge on each side forming anterior walls of large 

 transverse sockets for teeth ; sharp prominent concentric thickened semicircular 

 ridge situated at about half length of valve, bilobed and sharply indented in middle, 

 bounding pair of large depressed concave areas separated by thin short median 



* Ruedemann, Bull. 49, New York State Mus., 1901, p. 19, pi. i, figs. 8-20. 



