SYNOPSIS OF THE EECENT AND TEKTIAKY LEPTONACEA 

 OF NORTH AMERICA AND THE WEST INJ)IES. 



By William H. Dall, 



Curator, Division of Molluska. 



Having been engaged in reviewing the Tertiary species of this group 

 it was found necessary to investigate the standing of the genera and 

 higher groups, both recent and fossil, as the synonymy was found in 

 great confusion. The full details of this work will appear in Transac- 

 tions ' of the Wagner Institute of Science, Philadelphia, but it seemed 

 desirable to give a synopsis of the American species, with the revised 

 synonymy, for the use of students who might not have access to the 

 larger paleontological work. No doubt more thorough exploration of 

 our Southern coasts and the Antilles will add largely to the i^resent list, 

 which nevertheless adds several genera and some 20 recent species to 

 those heretofore known from our fauna. 



The Leptonacea form a very interesting and puzzling group. Their 

 characters combine features, characteristic in other Teleodonts of imma- 

 turity, with such as are more probably due to environmental modifica- 

 tions. Without being in themselves prototypes they exhibit features 

 which we may readily suj)pose might have been characteristic of proto- 

 typic Teleodonts. Groups which are really starting points for numerous 

 subsequently developed genera, are usually notable for their tendency 

 to vary and interchange characters. In the present case i)erhaps the 

 very general habit of commensalism, or parasitism, has produced 

 degeneration accompanied by a revival of atavistic primary characters. 

 The fact that authors, struck by similarity of dental features to those 

 of immature specimens of genera of widely different origin, have too 

 hastily referred species of Leptonacea to such families as the Mactridfe 

 or Cyrenidai is significant in this connection. 



It must be confessed at the outset that our knowledge of the anatomy 

 of recent Leptonacea is lamentably deficient. We have to assume 

 (which is never safe) that forms with similar dentition are generally 



' Volume III, Pt. 5. 



Proceedings U. S. National Museum, Vol. XXI— No. 1 177. 



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