152 PROCEEDINGS OF THE MALACOLOGICAL SOCIETr. 



and at the same time by the fact that none of them could actually be 

 referred to the genus Dosinia as usually defined. I was thus led to 

 study the recent forms of that genus more particularly, and found 

 that, while the great majority of species agree with the definition 

 given in conchological manuals, there are a few species which do not. 

 Of these the most striking and abnormal is D. trigona (Keeve), which 

 diilers in the following respects : — 



The shell is not orbicular, but ovately and bluntly trigonal, the 

 fore-part of the shell being well developed, so that the umbones are 

 subcentral. It is not compressed, but decidedly ventricose. The 

 lunule is not impressed, but large and superficial. There is no 

 escutcheon. The hinge differs in correlation with the anterior 

 development of the shell, the anterior lateral being strong, erect, and 

 distant from the anterior cardinal ; similarly, in the right valve there 

 is a deep and distant pit for its reception. The postero-dorsal margin 

 of both valves is grooved for a long distance to receive the bevelled 

 edge of the opposite valve. The pallial sinus is not elongate, but 

 rather short, wide, and rounded. 



Fig. 1. — Dosinia [Sinodia) triffona (Reeve). SligMly enlarged. 



There are some other species which resemble D. trigona to a greater 

 or less extent : these are D. tripla, Roem. ; D. derupta, Roem. ; 

 D. spharicula, Roem. ; and to a less degree B. excisa, Chem., and 

 B. suhtrigona, Sow., which are more nearly orbicular. 



Now if the well-known Oligocene shell which goes by the name of 

 Cytherea incrassata be carefully examined, it will be seen that it 

 resembles I), trigona in all the points above mentioned, and that it 

 further resembles Dosinia in the thickness of the shell and in the 

 massive hinge-plate, in the occasional rugosity of the anterior lateral 

 tooth, in the manner in which the right posterior cardinal springs 

 from the end of the incurved anterior margin, and also in the long 

 narrow pedal scar which runs under the hinge-plate below the two 

 anterior teeth of the left valve. 



