904 DR F. R. C. REED ON THE 



hinge-area narrow and nearly in plane of valve ; interior with short small strongly 

 bilobed cardinal process (the oval faces of the lobes facing inwards) rising from 

 small thick transverse hinge-plate; crural plates long, thin, slightly arched back, 

 very divergent, making angle of 10°-20° with cardinal line. Surface of shell 

 covered with very numerous fine straight rounded closely placed subequidistant 

 thread-like riblets, of which 30-40 are usually primaries, with 2-3 finer thread-like 

 lines between them increasing by intercalation to 6-12 on margin, every third or 

 fourth of these finer lines usually becoming rather larger near margin, and forming 

 secondary riblets shorter than the primaries ; all covered by close concentric striation. 

 Minute closely set pustules generally distributed over interior, fusing into sinuous 

 radial lines near cardinal angles. 



Dimensions. — Length, 21 mm. ; width, 33 mm. 



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



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



Remarks. — This shell is especially allied to Strophomena incurvata, Shepard 

 (=Str. Jllitexta, Hall) of the Trenton Formation in America. But the British 

 Strophomena simulans, M'Coy,t is also a near relative, though the muscle-scar in 

 the pedicle-valve is of a different character, and the brachial valve is more convex 

 and there are concentric rugse near the hinge-line. The Stinchar Limestone specimens 

 seem to have the pedicle-valve rather flatter than those from Balclatchie, and usually 

 all the radii are of subequal size, but occasionally the median primary riblet is 

 enlarged and conspicuous ; the muscle-scars also in the pedicle-valves from Craig- 

 head are less strongly marked. The Balclatchie examples, which are taken as typical. 

 consist of internal casts and external impressions of both valves and arc fairly 

 abundant. The relative strength of the primary and secondary radii vary, ami 

 they may be almost equal in size. 



Strophomena shallochensis, Davidson (emend.). 



(Plate XIX, figs. 11, 12.) 



1883. Strophomena sliallockiensis, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Sttppl., p. 192, 

 pi. xi, figs. 20, 21 (non pi. xii, fig. 30) ; pi. xvi, fig. 8. 



This species was somewhat briefly described by Davidson, and though he un- 

 doubtedly regarded the Shalloch Mill form as the type of the species, yet he figured 

 first (op. cit., pi. xi, figs. 20, 21) a shell from Corwen, North Wales. In the Shallocl 

 Mill type specimen (op. cit., pi. xvi, fig. 8) it should be remarked that in both va 

 the hinge-areas are steeply inclined to the plane of the valve, but in the case of the 

 concave pedicle-valve the hinge-area is at right angles and larger; there is also a large 



* Shepard, Amer. Journ. Sc, vol. xxxiv, 1838, p. 144, figs. 1, 2 ; Hall, Palseont. New York, vol. i, 1817, p. Hi. 

 pi. xxxiB, fig. 3 ; Hall and Clarke, Palxont. New York, vol. viii, Brach., i, pi. ix, figs. 1-7, pi. ixA, figs. 11-14; 

 Schuchert, Bull. U.S. Geol. Sure, No. 87, 1897, p. 431. 



t M'Coy, Syn. Brit. Palxoz. Foss. Woodw. Mm., p. 403, pi. iH, figs. 33, 34, ?35 ; Davidson, op. cit., vol. iii, 

 p. 297, pi. xlii, figs. 9, 9a-c, ?10. 



