STEOPHODONT^ OP THE UPPER HELDERBERG GROUP. 88 



middle of the valve, the width of the shell is greater than at the hinge- 

 line. The area is comparatively wide and much elevated in the centre ; 

 the hinge-line strongly crenulated. Muscular impressions comparative- 

 ly large and deeply marked, separated by a rather wide groove which 

 reaches more than halfway to the base of the shell. 

 The entire margin of the valve (in the casts) is elevated in a narrow- 

 band, within which it is abruptly depressed ; and the space between the 

 callosity and the muscular impression is marked by punctate vascular 

 impressions. 



In five well-marked individuals, like fig. 9 a, 6, of Plate xx, the casts are rarely more 

 than half an inch in length, and are readily recognized by their great convexity, nar- 

 row elevated border, and comparatively short hinge-line. 



A dorsal valve found associated with the ventral valves is represented on Plate xr, fig. 4, 

 which is enlarged from a mould in the gritstone : it has the margin abruptly recurved, 

 with a callosity extending along the border. The cardinal process is slender and bi- 

 furcate. 



In comparing a larger number of specimens, it appears that the well-marked 

 S. callosa may pass by several gradations into another form which is even more 

 numerous in individuals than the form described. In the absence' of the shells, it 

 may be impossible to decide satisfactorily ; but it appears not improbable that 

 the S. callosa (as originally described ) is an extreme variety of a more regularly 

 convex shell which is proportionally shorter, and sometimes possesses the callo- 

 sity near the margin in a moderate degree, but is often free from such marking. 

 The prevailing form is the following ; 



Stropliodonta callosa, var. 



PLATES XI & XV. 



Shell semielliptical. Ventral valve regularly convex, the greatest con- 

 vexity about the middle of the shell : hinge-line straight, strongly 

 crenulated, varying from a little less to a little more than the width of 

 the shell ; apex elevated above the hinge-line ; area of moderate width. 

 In the casts there is a narrow longitudinal mesial depression, often 



reaching more than halfway to the base ; the apex is distinctly bilobed ; 



the marks of the occlusor muscles make two well-defined prominences ; 



