FREE INSTITUTE OF SCIENCE 



TERTIARY FAUNA OF FLORIDA 



891 



in its possibilities ; and the other where the separation, being complete, is 

 made final by the development of a septum. In accordance with this hypoth- 

 esis we find the oldest fossil Mactridce spisnloid and even without a base of 

 attachment for the ligament proper ; later this base is supplied and certain forms 

 took on the Mactroid type. Ptcropsis is the earliest known genus of its sub- 

 family and is spisuloid, while the recent Raeta is Mactroid. The older /,//- 

 traria and the newer Trcsi/s, the older Standclla and newer Heterocardia, offer 

 parallel cases. The Zenatiince appear to be a relatively modern type and they 

 are all spisuloid, as is Anatindla. In each case the Mactroid features belong 

 to the modern or later stage of each group when they are present at all, some 

 groups not having yet acquired the requisite equilibrium. The Mesozoic forms 

 so far known have the Cyinbopliora hinge, the Eocene ones in America are 

 spisuloid ; total submergence in Miilinia and Rangia (derivatives from the 

 spisuloid type) comes later on in the Miocene, while none of the aberrant 

 latest developments has reached a Mactroid stage. These features may be 

 tabulated as follows : 



LlCAMKNT. 



Mactrina- . 



SPISULOID. 



SUBMERCF.I). 



MACTROID. 



Pteropsidiruc . 



I'teropsis. 



Labiosa. 

 Rattta. 



Ractella. 



Lutrariinae 



Lutraria. 

 Standclla. 



F.nstonia. 



Tresus. 

 Heteroi- \nli.i. 



Zenatiina: 



Zenatia. 

 Resania. 

 Darin a. 



Anatincllin.i- 



Anatinclln. 



