164 PROCEEDINGS OF THE MA.LA.COLOGICAL SOCIETY. 



radiating ribs, of which some are divaricate, in having apparently 

 three divergent cardinal teeth and possiblj^ an anterior lateral, in the 

 entire absence of any escutcheon, and in the deeply sunk ligament ; 

 also in the crenulation of the valve margins, and in the absence of 

 a pallial sinus. It differs from Crista in not having the large anterior 

 expansion of the hinge -plate bearing the lateral teeth, in not having 

 a distinct posterior cardinal separated by a groove from the nymph, 

 and lastly in the divaricate ribs being on the anterior side, while in 

 Crista they are on the postero-dorsal side ; on that side of Ptychomya 

 there is only a series of short tubercular ridges. 



On the whole I incline to think that the resemblances outweigh the 

 differences, but until the existence of an anterior lateral tooth is 

 established the affinity with Crista must remain doubtful. 



11. SuNETTA and subgenus Meroejsta. PI. VI, Fig. 8. 

 The existence of the genus Sunetta as far back as early Eocene time 

 was first pointed out by M. Cossmann in 1886, who referred four 

 species occurring in the Parisian Eocene to this group, though S. multi- 

 sulcata is probably only a variety of S. semisulcata, Lam. He does 

 not, however, seem to have noticed that these early forms differ from 

 all recent species in having smooth inner margins instead of crenulated 

 edges. Moreover, the dorsal border of the Eocene shells is not so 

 deeply inflected, so that the ligament is not so deeply sunk, and the 

 upper angle of the escutcheon is more obtuse. The hinge is similar to 

 that of the recent species, except that the posterior cardinal of the 

 right valve is deeply grooved, whereas in recent species it is narrow 

 and entire, or very feebly grooved. Lastly, the Eocene shells are much 

 less inequilateral, the umbones being subcentral and the shell 

 subtrigonal. 



Fig. 2. — Sunetta {Meroena) trigonula, Desb. Ovate variety; from a specimen in 

 the British Museum. 



These differences are certainly of sufficient importance to deserve 

 recognition, and are all of them more important than the smoothness 

 of the outer surface and more rounded shape, for which Jousseaume 

 has proposed to separate one or two recent species under the name of 



