STROPHODONT^E OP THE UPPER HELDERBERG GROUP. 85 
Stropliodonta parva. 
PLATES XI & XV. 
Strophodonta parva : Hall, Sixteenth Report on the State Cabinet, p. 37. 1863. 
Shell small, subhemispheric, a little wider than long : hinge-line equal¬ 
ling or shorter than the width of the shell; cardinal extremities 
rounded. Ventral valve gibbous : umbo elevated above the area-line; 
apex incurved; area linear. 
Surface marked by few rounded or subangular costae, which are simple 
on the upper part of the valve, but each one dividing into two or three 
above, or in the middle of the shell, and are often farther divided on the 
margin; those of the ventral valve often increase by subdivision, 
while those of the dorsal valve increase by intercalation. The larger 
striae are covered by fine undulating or interrupted longitudinal striae, 
and are sometimes marked by a sharp elevation along the middle. 
The casts of the interior of the ventral valve rarely show the muscular 
impression with much distinctness; the surface being marked as if by 
fascicles of fine striae, with sometimes a sharp elevated one between. The 
apex is bilobed; the occlusor impressions small and well marked, and 
the divaricator muscular imprints are broad and spreading. 
Impressions of the exterior of the dorsal valve are common, and are 
readily distinguished by the strong subangular costae on the upper part, 
while they become grooved and increase by intercalation on the lower 
part of the mould. 
In exfoliated specimens, the surface has the aspect of those with fa¬ 
scicles of striae and sharp elevated striae between. 
The form and proportions of the species seem to be pretty constant, 
rarely exceeding and usually less than half an inch in width. The ordinary 
condition of the specimens is that of casts, or of impressions of the ex¬ 
terior shell; while in the more calcareous part of the rock, specimens 
preserving the shell have been obtained. 
