922 DR F. R. C. REED ON THE 



Scenidium Leivisi, Davidson, var. nov. craigensis. 

 (Plate XXII, figs. 4-9.) 



Shell small, transversely triangular to semielliptical, sharply folded down middle 

 line ; hinge-line equal to width of shell ; cardinal angles subacute to rectangular. 

 Pedicle-valve deeply conical, strongly carinate, with dependent lateral lobes making 

 a wide angle ; hinge-area large, triangular, nearly at right angles to plane of valve ; 

 beak small, prominent, forming apex of cone, pointed, elevated, and slightly 

 incurved. Brachial valve bilobed by deep angulated median sinus increasing in 

 width anteriorly, with gently convex lateral lobes ; anterior margin more or less 

 emarginate in middle ; beak inconspicuous ; hinge-area narrow ; interior with long 

 linear cardinal process, median septum about half the length of valve, and pair of 

 short converging plates uniting with septum and forming spondylium. Surface of 

 valves covered with 30-40 simple straight subangular closely placed regular equal 

 fine ribs ; concentric striae very faint or absent. 



Dimensions. — Length, 5-6 mm. ; width, 7-8 mm. 



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



Localities. — (l) Craighead; (2) Ardmillan, Dow Hill, Balclatchie conglomerate. 



Remarks. — This small variety is very closely similar to the Wenlock type-form.* 

 It differs from var. asteroideum in the greater fineness and number of the ribs and 

 absence of imbricating striae, and from var. woodlandense in its more triangular shape, 

 and finer regular ribs. In shape the shell much resembles Orthis melmerbiensis, Reed,t 

 from the Dufton Shales. 



Scenidium Leivisi, Davidson, var. woodlandensis, Davidson (emend.). 



1883. Scenidium Lewisi, var. woodlandiense, Davidson, Mori. Brit. Foss. Brach., vol. v, Silur. Suppl., 

 p. 174, pi. xi, figs. 1, 2. 



As Davidson remarks, this shell " partakes of all the essential characters of the 

 Wenlock type, differing from it in size only, being nearly twice as large." It attains 

 a width of 13-14 mm. The interior is imperfectly known. This variety differs also 

 from the Craighead variety by its larger size and more transverse shape, and 

 by the, pedicle-valve being less strongly carinate and without any distinct median 

 fold ; the beak also is higher and more incurved, the hinge-area lies nearly in the 

 plane of the valve, and the ribs are coarser and less regular, increasing by intercala- 

 tion and rarely by division. It seems more allied to the variety Hughesi, Davidson 

 (op. cit., p. 254, pi. xxxviii, fig. 26). 



Dimensions (type, op. cit., pi. xii, fig. l). — Length, 6 "8 mm. ; width, 8 '5 mm. 



Horizon. — Saugh Hill Group. 



Localities. — Woodland Point, Newlands. 



* Davidson, op. cit., vol. iii, p. 208, pi. xxvi, figs. 4-9. 



t Reed, Geol. Mag., dec. v., vol. vii, 1910, p. 296, pi. xxiii, figs. 4-8. 





