898 DR F. R. C. REED ON THE 



Stropheodonta ? Nicholsoni, sp. nov. 

 (Plate XVII, fig. 21 ; Plate XVIII, figs. 1-8.) 



L883. Strophomena imbrex, var. semighbosina, Davidson (pars), A/ore. Brit. Foss. Brack., vol. v, StYar. 

 Suppl., p. 195, pi. xv, figs. 9, 10 (non fig. 11). 



Shell concavo-convex, compressed, subquadrate to semielliptical, widest along 

 hinge-line, about one-third wider than long ; cardinal angles subrectangular to sub- 

 acute, less than right angles, sometimes slightly alate and pointed at 60°-75° 

 [hinge-line furnished along whole length with coarse projecting denticles not closely 

 placed]. Pedicle- valve inflated, strongly convex, with slightly flattened lateral 

 angles ; beak obtuse, somewhat swollen, rather high and incurved ; hinge-area 

 triangular, transversely striated, lying in plane of valve, with large triangular del- 

 thyrium. Interior of pedicle-valve with pair of stout triangular teeth and weakly 

 impressed subcircular muscle-scar less than one-fourth length of valve and composed 

 of pair of short blunt flabelliform diductors separated by narrow oval adductors. 

 Brachial valve concave, usually with broad shallow indefinite median depression ; beak- 

 small, not swollen, inconspicuous ; hinge-area narrower than that in opposite valve 

 and nearly at right angles to it ; interior with strong short crural plates inclined 

 at 15°-20 u to hinge-line ; cardinal process imperfectly known ; muscle-scars very 

 faint ; weak concentric submarginal rugose and coarsely pustulose internal thicken- 

 ing sometimes present. Surface of valves covered with 60-100 straight subangular 

 or rounded radiating subequal riblets, mostly primaries, closely placed, with shorter 

 and usually rather smaller secondary ribs occasionally intercalated at irregular 

 intervals towards margins ; interspaces equal in size to ribs, subangular to rounded; 

 all ribs and interspaces crossed by fine regular concentric lineation ; ribs apparent 

 on interior of both valves. 



Dimensions. — 



(1) (2) 



Length . . . . . .21 11*0 mm. 



Width 30 15"5 mm. 



Horizon. — Balclatchie Group (including conglomerate). 



Locality. — Balclatchie. 



Remarks. — The cardinal process of this shell is unfortunately not well preserved in 

 .any of the specimens ; its apparent simplicity and absence of bifurcation is probably 

 due to the fact that only the stalk is preserved in the few imperfect casts or impres- 

 sions which are available. The cardinal denticulation also is somewhat uncertain. 

 The shell has altogether a certain Orthoid appearance which renders its true position 

 doubtful, though its general characters seem to point to Stropheodonta. 



The specimens from the Balclatchie conglomerate referred by Davidson to ' Stro- 

 phomena imbrex, var. semiglobosina," were inaccurately figured, for in one (op. cit., 

 fig. 1 0) the cardinal angle on the left side is omitted and the long straight hinge lii 

 therefore unduly shortened. In the other specimen (op. cit., fig. 9) the cardinal angle- are 



