176 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY, 



Of these various groups only six or seven occur in the Cretaceous 

 rocks of England and France ; these are Dosiniopsis, Callistina, 

 PtycJiomya, Cyclorisma, Flaventia., Baroda, and possibly Calpitaria. 

 It will be noticed that none of these has any modern representatives, 

 unless possibly in the case of Calpitaria ; in other words, all (or nearly 

 all) the Cretaceous shells belong to groups that have become extinct. 



In the Eocene, however, we find several genera and subgenera 

 which are certainly identical with recent groups ; these are Callista, 

 Macrocallista, Pitaria, Circenita, Gouldia, dementia, and Tapes, while 

 Meroena is not far removed from Sunetta. 



In the Oligocene three other groups make their appearance, viz., 

 Sinodia (if not represented in the Eocene), Circe, and CMone ; while 

 in Meretrissa we have a near approach to Meretrix. 



There are a few groups which do not seem to have continued to 

 exist beyond Eocene or Oligocene times ; these are Atopodonta, 

 Aphrodina, Tivelina, DoUfusia, Cyprimeria, Mercimonia, Psathura, 

 Textivenus, and Venerella ; unless the last named is still represented 

 by a West American shell which was described by P. P. Carpenter 

 under the name of dementia suhdiaphana, but is referred to Venerella 

 by Dr. W. H. Dall. 



Another point which a study of these fossil forms has pressed upon 

 my attention is the more generalized and less difPerentiated facies of 

 the cognate Eocene groups. This is perhaps most apparent in the 

 case of Callista and Pitaria ; thus several species, which by their 

 hinge seem to belong to Callista, have the external surface and the 

 shorter ascending pallial sinus of Pitaria, and I have therefore been 

 compelled to establish an intermediate group for their reception. 



Again, in Tivelina we seem to have a group of shells which has 

 branched off from the common ancestor of Callista and Pitaria, for in 

 some of the species the hinge resembles that of Callista, in others it is 

 like that of Meretrix, and in some it makes a near approach to Pitaria. 

 Tivelina seems to have been a plastic group, i.e. one which had a 

 special tendency to develop variations while still retaining a certain 

 general facies ; thus it varies in shape, in surface sculpture, and in 

 depth of sinus, as well as in the hinge-characters. 



With regard to the peculiar minute discontinuous radial striae which 

 are a special feature of the recent genera Callista and Meretrix, th.eBe 

 are plainly visible on good specimens of Chionella ovalina and Macro- 

 callista Icevigata, but they do not seem to be present in Aphrodina 

 nitidula, for, though the surface was certainly smooth and may have 

 had a vernicose periostracum, I have not been able to detect any such 

 striae even in the best-preserved specimens. 



Finally, it may be noted that in Meroena we seem to have a link 

 between Callista and Sunetta — the smooth valve-margins, the more 

 obtuse escutcheon - borders, and the breadth of the posterior tooth, 

 marking a less degree of differentiation ; and in good specimens of 

 M. trigonula I have detected the characteristic ingrained striae of 

 Callista. 



