888 DR F. R. C. REED ON THE 



Pedicle-valve convex, most so in middle ; beak moderately incurved, somewhat 

 swollen, rounded and projecting; hinge-area in plane of valve; interior- of shell 

 rugose, with small short ill-defined diductors (apparently not elongated nor divergent 

 as in type of species) ; and with pair of strong slightly divergent vascular trunks 

 arising from beak, traversing diductor scars, and dividing at about one-third the 

 length of valve into several subparallel finer branches. Brachial valve concave, with 

 interspaces on surface between radii slightly depressed ; interior with pair of laro-e 

 elongate linguliform deeply sunk muscle-scars diverging slightly ; median interspace 

 between muscle-scars thickened and bearing low median septum ; each muscle-scar 

 divided subequally by strong longitudinal ridge. Interior of both valves marked 

 with close broken radial lines composed of more or less fused pustules which become 

 larger and more distinct near cardinal angles. External surface of shell ornamented 

 with 7-11 primary radii, with numerous fine radial striae in interspaces. 



Dimensions. — Length, 10'5 mm. ; width, 21 '4 mm. 



Horizon. — Mulloch Hill Group. 



Locality. — Mulloch Hill. 



Remarks. — The internal characters of the pedicle-valve differentiate it from the 

 type form. The presence of cardinal denticulations, though represented by David- 

 son * in the figure of a specimen from the Pentland Hills (but not mentioned in his 

 diagnosis of PI. transversalis, and apparently absent in typical Wenlock examples), is 

 another characteristic feature. 



Plectambonites transversalis, Dalman, var. nov. penkillensis. 

 (Plate XVI, figs. 3-7.) 

 1883. Leptxna transversalis, Davidson (pars), Mon. Brit. Foss. Brach., vol. v, Silur. SuppL, p. 227. 

 Shell transversely spindle-shaped to subtriangular ; cardinal angles produced, 

 acutely pointed at 30°-45°. Pedicle-valve convex, subcarinate, with median portion 

 strongly swollen and lateral portions more or less depressed ; beak strongly in- 

 curved, projecting, somewhat swollen ; hinge-area narrow, triangular, lying in plane 

 of valve. Interior of pedicle-valve radially pustulate, with pair of small teeth and 

 pair of elongaated slightly divergent diductors deeply sunk posteriorly but weak and 

 undefined anteriorly ; short low thick median septum in front of them ; pair of 

 strong straight vascular trunks traversing diductors longitudinally, diverging slightly 

 anteriorly, and dividing at about one-third length of valve into 3-4 smaller radiating 

 branches of unequal length. Brachial valve concave ; interior unknown. Surface of 

 pedicle-valve marked with 8-10 primary radii, the median one occasionally stronger 

 and more prominent than rest, with a secondary radius of about half the length and 

 numerous fine radial striae in each interspace. 



Dimensions. — Length, 8-11 mm. ; width, 12-15 mm. 

 Horizons. — (l) Penkill Group ; (2) Camregan Group. 



* Davidson, op. cit., iii, p. 318, pi. xlviii, figs. 9a, 96. 



