880 . DR F. R. C. REED ON THE 



composed of two subtriangular pointed diductors, anteriorly free, enclosing posteriorly 

 a short broadly oval adductor scar ; interior marked by pair of subparallel equal 

 vascular trunks arising from muscle-scar and by numerous smaller radial vessels 

 dividing once or twice ; a strong lateral trunk arises also from beak and runs 

 outwards parallel and close to cardinal line ; some large elongated pustules lie 

 sparsely scattered between the vascular branches. Brachial valve gently concave ; 

 beak inconspicuous ; hinge-area narrow, steeply inclined to plane of valve ; interior 

 with prominent trilobate cardinal process on large thickened subpentagonal hinge- 

 plate ; crural plates strong, divergent ; pair of large contiguous flabelliform deeply 

 sunk muscle-scars, nearly half length of valve, each weakly bilobed in front and 

 obscurely divided into outer and inner halves ; thin short median septum present, 

 not continued in front ; rest of interior with close subparallel small vascular trunks 

 running to margins of valve, the pair in front of inner adductors being generally 

 rather stronger. Surface of shell marked with about 10 equidistant fine primary 

 radii, often increased to double the number by secondary ones; 8-12 delicate radial 

 strise in each interspace, and minute p-unctse in radial lines. 



Dimensions. — Length, 10-12 mm. ; width, 18-20 mm. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Remarks. — This species is fairly abundant in the Starfish Bed. Its internal 

 characters differentiate it from all the other forms. Professor 0. T. Jones' 

 description and figures of this new species have not yet been published, though 

 a MS. name was attached by him to several of Mrs Gray's specimens some 

 years ago. 



Plectambonites scissa (Salter). 

 (Plate XIV, figs. 34, 35.) 



1865. Leptsana scissa, Salter MS., Cat. Coll. Mus. Pract. Geol., p. 36. 



1870. Leptxna scissa, Salter, Davidson (pars), Mon. Brit. Foss. Brach., vol. iii, p. 325, pi. xlvii, 

 figs. 21-23 (non figs. 24, 25). 



In the Girvan shells referable to this species, as in Davidson's figured example 

 (op. cit., fig. 22a) from the Llandovery, the muscular scars in the brachial valve are 

 large, triangular, flabelliform, deeply sunk (especially in front), with sharp raised 

 lateral edges united anteriorly across the middle of the valve by an acutely emarginate 

 continuous sharp ridge joining the low broad rounded median ridge which separates 

 the anterior ends of the slightly divergent scars. Though these scars are radiately 

 striated, yet they do not show clearly a division into conjoint adductors, nor is there 

 any marginal bilobation. A thin low thickening of the shell just inside the margin 

 is usually developed in this valve as in PI. llandeiloensis. Davidson described 

 the muscular characters of the brachial valve from an Ordovician Sholeshook 

 specimen (op. cit, fig. 25), which he wrongly attributed to the same species as Salter's 

 Llandovery type, the true PI. scissa. In the pedicle-valve of these Girvan repre- 



