2:58 



front ; sides very steeply iuclined, top rounded and flattened, with usually a 

 very shallow, longitudinal furrow, exceedingly narrow at the beak, but broad- 

 ening and disappearing on approaching the front. The summit of the fold, 

 from the beak to the front, describes a moderately strong curve, which tends to 

 become more or less straight towards the front. The elevation of the fold in 

 front is quite variable. 



The test has, on each side of the fold and sinus, 10 to 16 simple, rounded, 

 prominent plications, the reverse plications being of the same form but nar- 

 rower. On the ventral valve, the plications bordering the sinus are sometimes 

 slightly larger than the others, and are well defined up to the beak. Towards 

 the sides they gradually decrease both in width and prominence, sometimes 

 dying out entirely on the cardinal angles, which are thus frequently left 

 smooth, as is also a narrow space extending just in front of the cardinal mar- 

 gins, to within a varying distance of the beak. At the sides of the fold in the 

 dorsal valve, the plications arch rapidly from the beak, curving strongly to the 

 front, but less and less so as the cardinal angles are approached, where the 

 valve is more or less flattened, the plications diminishing in prominence 

 towards the sides as in the ventral valve, but seldom leaving the cardinal 

 angles smooth. Sometimes the valves are marked, usually towards the front, 

 by one, two or three, seldom four, prominent lines of growth, and some im- 

 pressions of the fold and sinus show indications of many fainter ones. 



The dental plates of the ventral valve are thin, divergent, generally very 

 short, though in the older specimens they sometimes extend forward nearly 

 one-third the length of the valve, each including two or three plications be- 

 tween itself and the sinus. 



The specimens vary much in dimensions, one large one measures, length, 15 

 m. m., breadth, 36 m. m., depth, 13 m.m.; another, 15, 45 and 12 m. m. 



The test must have been a thin one, for the exterior markings are very 

 plainly impressed upon the inner mould. 



This species belongs to tlie group of S2nrife7'ae,\\ii\\ broad hinge 

 area and more or less extended sides, which is so common in the 

 Devonian ; but the collections at command for comparison have 

 been so meagre that its relations to other species have been but 

 imperfectly determined. It resembles closely >S'. varicosa of the 

 Corniferous limestone, from which, however, it differs in the greater 

 number of plications, which are not angular, and also in the nar- 

 rower hinge area. Many of the smaller and more mucronate vari- 

 eties approach 8. angusta of the Hamilton group in shape, but in 

 the latter species the plications are smaller and more numerous. 

 From the European Devonian S. sul-cuspidata, Sclmur, it differs, 

 among other features, in the much narrower hinge area. 



