204 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



"which several of the more distinct types of the present fauna may be 

 traced to their inception. 



These facts I hope to fully exemplify in tlie final part of my 

 "Tertiary jMollusks of Florida," wliich will probably be ready for 

 publication in the course of the next year. 



I have therefore decided to offer to the Society a table of the 

 resultinsf classification, p;iving the families, sub-families, and f!:encra 

 of the Mactracca, together with the types upon which the subdivisions 

 are founded ; accompanied by a statement of the characters of the 

 families and sub-families, and of the nomenclature I have adopted for 

 the diiferent parts of the hinge. This will enable me to receive 

 the criticisms of members and others interested, and thus perfect 

 my final arrangement, which will be adequately illustrated. 



To those whose knowledge of the group is chiefiy gained from 

 existing text-books I fear the first impression will be that an undue 

 multiplication of subdivisions has been proposed. As a matter of fact, 

 I have suppressed about as many subdivisions as I have adopted, and, 

 if anyone will attempt to state in tabular form the characteristics 

 of a full series of species, it will be found, I believe, that the sub- 

 divisions here adopted are an aid to clear comprehension of the 

 various modifications, and enable relationships of form and geo- 

 graphical disti'ibution to be comprehended in a manner impossible 

 under the old classification. 



The hinge of the gi'oup, which I have called Teleodesmacea, including 

 the majority of recent Pelecypods, is characterized by an alternation 

 of the teeth of the two valves, so that, when the valves are open, the 

 series in each is composed of teeth separated by spaces, into which 

 those of the other valve fit when closed. Shells with this arrange- 

 ment have been named Heterodonta by Neumayr. He referred the 

 typical Mactracea to another group which he characterized as having 

 vacant spaces in one valve which no tooth exists in the other valve to 

 fill, and which group he named Desmodonta. Thus, if the inferior 

 A-shaped tooth of Mactra be regarded as a coalescence at the top of 

 two originally separate teeth, the space between the arms receiving 

 no tooth from the opposite valve would conform to the Dcsmodont 

 definition. But, as I have shown in my recent " Monograph of the 

 genus Gnafhodon'^ \_ — Jian(/ial^,^ and more fully in my forthcoming 

 Florida report, this tooth is really a single tooth, dynamically modified 

 to fit the superior A-shaped cardinal, and the latter is really composed 

 of two primitive (and still not always coalcscent) teeth, and therefore 

 the hinge is of the Heterodont, and not of the Dcsmodont, type.- 



To fully understand the mechanism and development of the Mactroid 

 hinge one must give a careful study to its minute details. It is only 

 when one finds charactei's, which would seem at first sight veiy 



1 Proc. U.S. Nat Mus. xvii. (1894) pp. 89-106. 



^ Bittucr has recently worked out the same conclusion in a critique on some of 

 Neumayr's views : Ueher die syst. Stellung von Mactra, Verb. k.k. geol. lleichsau- 

 stalt, 1892, pp. 232-240. 



