TRANSACTIONS OF WAGNER 

 962 



'TERTIARY FAUNA OF FLORIDA 



of the laterals may be obsolete or practically absent. The anterior right dorsal 

 margin is often distinctly grooved in front of the laterals, to receive the edge 

 of the opposite valve. The posterior end of the shell is shorter than the 

 anterior ; there is no distinct lunule ; the limits of the posterior truncation 

 are often sharply angular and carinate, the sculpture of this part of the shell 

 is generally more emphatic than that of the anterior region. There is some- 

 times a well-defined escutcheon, limited by a rib or ridge. In such cases the 

 sculpture on the escutcheon differs more or less from that outside of it, and 

 the limiting rib may appear on the distal margin as a prominent tooth or 

 projection. The ventral and distal shell margins are usually sharply fluted; 

 even those species which are said to have entire margins will show under 

 magnification, in well-developed specimens, a fine serration on at least part 

 of the edge. The nepionic shell of Donax exhibits a provinculum and three 

 primary cardinal lamellae in each valve, of which one eventually is smothered 

 by the growth of the resiliar nymphs, except in Macharodonax. It is doubt- 

 ful whether those forms like D. ovalinus Lamarck, which have been referred 

 to Heterodonax, are congeneric with H. bimaculatus, the type of that 

 genus. They have much more resemblance to the section Latona, but 

 doubtless the question can be decided only by a comparison of the anatomical 

 features. 



The group may be arranged as follows: 



Genus Iphigenia Schumacher, 1817 (Donacina Per., 1821, and Procos Gistel, 



1848). Type Donax l&vigatq Ch. 



Shell large, subtriangular, subequilateral, without radial sculpture ; thick, 

 with entire ventral margins ; two cardinals, the larger bifid, in each valve and 

 two obsolete laterals in the right valve. 



Genus Egerella Stoliczka, 1870 (Egeria Lea, 1833, not Roissy, 1806). Type 



E. subtrigonia Lea. Eocene. 



Shell small, of variable form, with faint radial sculpture, thin, with serrate 

 ventral margins ; cardinals as in Iphigenia, but the laterals absent. 



Genus Donax (L.) Lamarck, 1799. Type D. trunculus L. 



Section Donax s. s. 



Shell elongate, smooth, with no posterior carination ; ventral margins with 

 obsolete serration ; cardinal teeth two in each valve, the larger often bifid ; 

 laterals both in the left valve, the anterior hardly distinguishable from the 

 margin, of which it is a sort of modification. 



