ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 879 



Remarks. — The flatness of the valves and shape distinguish it from the typical 

 PI. quinquecostata ; the cardinal angles are never acute, nor produced, nor semi- 

 cylindrical ; the ribbing is, however, more like that of this species than of PL 

 sericea or any of its varieties. 



Plectambonites quinquecostata (M'Coy), var. no v. sublobata. 

 (Plate XIV, figs. 20-24, 10.) 



Shell transversely oblong, with lateral edges nearly straight and subparallel and 

 at right angles to anterior margin ; cardinal angles subacute, pointed, not produced ; 

 anterior margin nearly straight or broadly and weakly emarginate ; surface of shell 

 weakly bilobed by wide shallow median depression generally holding a low rounded 

 median fold ; lateral lobes gently swollen and more elevated. Pedicle-valve with 

 more or less defined flattened disc, subgeniculate anteriorly ; beak small, scarcely 

 elevated, not projecting nor incurved ; lobation of surface usually distinct. Interior 

 unknown. Brachial valve strongly concave ; interior with long oval flabelliform 

 non-contiguous slightly divergent muscle-scars about half the length of shell, with 

 inner edges sharply raised ; anterior (inner) adductors better marked and narrower 

 than posterior (outer) adductors ; thin median septum separating muscle-scars ; 

 general surface of interior densely pustulate, in regular radial lines. Surface of shell 

 marked with 3 straight primary radii diverging at 45° from each other, with occasion- 

 ally 2-3 shorter weaker secondary radii present ; very fine delicate striae in interspaces. 



Dimensions. — Length, 7 "2 mm. ; width, 10*5 mm. 



Horizon. — Balclatchie Group (conglomerate). 



Locality. — Balclatchie. 



Remarks. — In the lobation of the shell we are reminded of Plect. segmentum, var. 

 cornuta, Dav., # from the Wenlock. The few primary radii are like Plect. quinque- 

 costata and its varieties. The interior of the brachial valve resembles Plect. sericea 

 in its wide sense. On the whole it seems best to associate it with PI. quinquecostata, 

 especially bearing in mind its resemblance to the variety balclatchiensis. 



Plectambonites ruralis, sp. nov. 

 (Plate XIV, figs. 25-33.) 

 Shell concavo-convex, transversely semicircular to semielliptical, about twice 

 as wide as long, widest along hinge-line ; cardinal angles about 75° or sub- 

 rectangular. Cardinal margin thickened, denticulated along most of its length with 

 minute oblique teeth. Pedicle-valve strongly and uniformly convex, lateral portions 

 not flattened ; beak small, incurved, not swollen nor projecting, slightly elevated ; 

 hinge-area triangular, narrow, in plane of valve ; interior with short broad trans- 

 versely subpentagonal emarginate muscle-scar, about one-fourth length of valve, 



* Davidson, op. cit., vol. v, Silur. Suppl, p. 166, pi. xii, figs. 1-3. 



