Tertiary Mollusks of Florida. 



BY W. H. DALL. 



PART III. 



A new Classification of the Pelecypoda, with an enumeration of the differ- 

 ential characters of the Orders, Suborders, Superfamilies, and Families, a state- 

 ment of their range in Geological time, and an enumeration under each family 

 of the chief generic groups believed to be referable to it. Preceded by a brief 

 discussion of the features of the Pelecypod organization chiefly available as 

 diagnostic characters. Introductory to the description of the Tertiary Pelecy- 

 poda of Florida. 



In May, 1889, a Preliminary Catalogue of the shell-bearing Marine Mol- 

 lusks and Brachiopods of the Southeastern Coast of the United States was 

 published by the U. S. National Museum as Number 37 of its series of Bulle- 

 tins. In that work, which, from unavoidable causes, was prepared in great 

 haste by the writer, a new classification of the Mollusca was proposed. Since 

 that time much new material has been examined by various naturalists, bear- 

 ing upon the Pelecypoda, and the writer has systematically studied the sev- 

 eral groups as opportunity offered, with the view of placing the resulting 

 classification upon a firm basis, derived from a consideration of the totality of 

 characters. The completion of this work, so far as our present knowledge 

 permits, has resulted in a revision of the earlier arrangement in many details, 

 and an apparent confirmation of the general principles upon which the earlier 

 system was founded. With the presentation of the data of the system now 

 adopted, the classification is submitted to the judgment of systematists for 

 criticism and amelioration. 



The bivalve molluscan shell is a defensive covering which, normally, pro- 

 tects the more delicate organs of the body with a permanent shield ; and, when 

 the valves are open, admits of the extrusion of organs connected with alimen- 

 tation, respiration and progression. When the valves are completely closed 

 they form, normally, a defensive armour against enemies and environmental 

 irritation, to the extent of their strength. Leaving out of consideration, mo- 

 mentarily, abnormal degenerate or exceptional modifications of the type due 

 to special circumstances, throughout the following discussion, except where 

 expressly mentioned, attention will be called to the fundamental type, the 



