STROPHODONTJS OF THE UPPER HELDERBERG GROUP. 
80 
Stropliodonta patersowi. 
PLATES XII & XIII. 
Strophomena ( Strophodonta ) patersoni : Hail, Tenth Report on the State Cabinet, p. 114. 1857. 
Shell semioval. Ventral valve convex, becoming gibbous in the middle, 
somewhat deeply deflected at the sides and front; sometimes marked 
by a few radiating undulations or folds, which affect both ventral and 
dorsal valves towards the front : hinge-line straight; extremities often 
salient. 
Surface marked by distant elevated radiating striae, and the intermediate 
spaces by undulating fine striae, and crossed by short abruptly arching 
wrinkles which are interrupted by the strong radiating striae. The 
finer radiating striae on the ventral valve are from three to ten or 
twelve between the stronger ones ; while on the dorsal valve the spaces 
are wider, and there are from ten to twenty finer striae between the 
stronger ones. The finer striae are crossed by equally fine concentric 
striae, giving the entire surface a delicately cancellated appearance. 
The ' short interrupted arching wrinkles vary extremely in strength; 
usually becoming obsolete towards the front of the shell, and in some 
specimens are scarcely‘or not at all perceptible. 
In young shells the cardinal extremities are very salient, the ventral 
valve moderately convex, usually becoming more so as the shell increases 
in size, but sometimes continuing of moderate convexity till more than 
half grown; but in such specimens, the margin is sometimes abruptly 
inflected. Older specimens become very gibbous, and sometimes longitu¬ 
dinally wrinkled. 
In one specimen partially preserving the shell (fig. 2, Plate xm), the 
muscular impression of the ventral valve is of the same form as in S. 
inequiradiata, but smaller. In a cast preserving the striae upon the margin, 
as in characteristic specimens of the species, the muscular impressions 
are likewise small, but exhibit the same form as the one figured. In a 
[ Paleontology IV.] 
12 
