84 Mr. J . Lycett on the Hinge of the Fossil Genus Platymya. 



similar kind of posterior elongated rib terminating anteriorly in 

 a tooth and opposite corresponding fossa so small as not to form 

 any projection beneath the hinge plate ; the internal moulds con- 

 sequently would exhibit little of the structure of the hinge, and 

 supply no sufficient data whereby its real character could be in- 

 ferred. The narrow lengthened posterior groove in each valve 

 resembles those in Mactromya, in which however an hiatus re- 

 mains between the grooves which does not exist in Platymya. 

 Ar corny a is destitute of these grooves. 



Example. Platymya Rodborensis. 



Shell compressed, subequilateral, with the posterior side wide, 

 truncated, gaping moderately, the anterior extremity being nearly 

 closed. The valves are equal, or with no apparent difference in 

 their convexity. An obtuse and rather indistinct keel passes 

 obliquely from the umbo to the infero-posterior extremity ; folds 

 or ridges concentric and irregular, distinct only upon the two 

 extremities of the shell, and passing over the keel bent nearly at 

 a right angle. Young specimens have their lateral diameter 

 comparatively greater, but like the adult shell the middle portion 

 is nearly smooth. The general outline has some resemblance to 

 P. tenuis (Etud. Crit. t. 10 a, fig. 5-6), but in that species the 

 hinge-line is more nearly horizontal, and the posterior border of 

 the shell has a much larger hiatus. Our shell has likewise 

 less convexity, and its posterior aperture is much smaller than in 

 P. hiantula (Etud. Crit. t. 10 a, fig. 7-13) ; to other species the 

 resemblance is more remote. 



In common with other species of the genus, the general form 



