FREE INSTITUTE OF SCIENCE 



873 

 TERTIARY FAUNA OF FLORIDA 



lively coalescent above. The anterior edge of the cartilage-pit is not con- 

 cerned with any of the teeth of Mactra except when they are accidentally 

 superposed upon it. The laterals also are not related to any part of the pit, 

 as such, though possibly they may have originated through plications not 

 unconnected with the ancestral proto-Iigament. In short, Neumayr was mis- 

 taken in supposing Macira to be a Desmodont, as sufficient material will show 

 any one. 



Family MACTRID^. 

 The family Mactrida, when divested of some extraneous matter, is 

 divisible into several groups from our present knowledge. These groups 

 will be regarded here as subfamilies, though, as in many other cases, it is by 

 no means determined whether their systematic value is precisely equivalent 

 to other groups of the same nominal rank. It suffices that they seem to be 

 natural groups, within the family, of higher than generic value. 



Subfamily MACTRIN.^. 

 Shell subequilateral, nearly closed ; hinge normal (as previously de- 

 scribed, p. 862), fully developed; siphons partially or wholly naked, wholly 

 retractile within the shell ; mantle, between siphons and anterior adductor, 

 chiefly open ventrally. 



Subfamily PTEROPSIDIN/E. 



Shell subequilateral, nearly closed, thin ; hinge feeble, concentrated, the 

 laterals partly obsolete or much reduced ; siphons wholly retractile, naked ; 

 mantle partially closed ventrally. 



Subfamily LUTRARIIN.S;. 



Shell inequilateral, widely gaping ; hinge tending to be irregular, the 

 laterals partly reduced or obsolete ; the chondrophore free, in the plane of 

 the hinge-plate; siphons contractile, not retractile within the shell, clothed 

 with a horny epidermis to their tips ; ventral opening of the mantle short, 

 and the foot correspondingly reduced in size. An opisthopodial orifice some- 

 times present. 



Subfamily ZENATIIN^. 



Shell inequilateral, compressed, thin ; hinge concentrated, irregular, the 

 laterals tending to become obsolete or absent; chondrophore bent out of the 

 plane of the hinge-plate and more or less adherent to the valve ; siphons 

 contractile, naked ; ventral opening of the mantle and foot variable. 



