STROPHODONT^ OF THE UPPER HELDERBERG GROUP. Stf 



Strophodonta parva. 



PLATES XI & XV. 

 Strophodonta parva : Hxll, 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 

 strise 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. 



