STROPHODONT^ OP THE UPPER HELDEKBERG GROUP. S9 



/ Stropliodonta patersoni. 



PLATES XII & XIII. 

 Strophomena (Strophodonta) patersoni : Hall, 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 afiect both ventral and 

 dorsal valves towards the front : hinge-line straight ; extremities often 

 salient. 

 Surface marked by distant elevated radiating strife, 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 striaa 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 xiii), the 

 muscular impression of the ventral valve is of the same form as in S. 

 inequiradiata, but smaller. In a mst preserving the striae upon the margin, 

 as in characteristic specimenFof the species, the muscular impressions 

 are likewise small, but exhibit the same form as the one figured. In a 

 [ Pai..«ontoloot IV.] 12 



