1916] Packard: Mesozoic and Cenozoic Mactrinae 



277 



By the close of the Neocene these forms became extinct. The genus 

 is now known on the Pacific Coast only from the Panama province. 



The Miocene was the period of the greatest generic and specific 

 differentiation of the family. Previous to that time the species 

 were few in number and relatively constant in shape. During that 

 period a considerable number of new forms appeared, most of which 

 were exceedingly variable, giving rise to a number of species, many 

 of which became extinct during the Pliocene. A few of the less 

 variable forms persisted through to the Recent. Specific differen- 

 tiation increased until the present fauna is characterized by clearly 

 defined species which are in general not closely related to one an- 

 other. 



CONCLUSIONS 



Of all the principal parts of the shell, the mactrine hinge is the 

 least variable within the limits of a single species. It varies some- 

 what as regards relative height or thickness of the teeth, but the 

 relative positions of the dental elements with reference to one another 

 are remarkably constant for a given species, and therefore serve as 

 important bases for specific classification. 



The known mactrine forms include five mactroid species, one of 

 which is here described for the first time. The genus Spisula is 

 represented by twenty-three species, six of which are new. The hinges 

 of eighteen of these have been studied. Three species and two vari- 

 eties of Mulinia are recognized, one of the species being considered 

 as new. 



SYSTEMATIC DESCRIPTIONS 



Superfamily MACTRACEA Gray, 1837 



Resilium situated on a chondophore ; bifid cardinal in the left 

 valve fits below the two cardinal arms of the right valve. 18 



Family MACTRIDAE Gray, 1837 



Shell trigonal, equivalve ; posterior gape more or less evident ; 

 umbones prosogyrate ; hinge plate well-developed, subtrigonal in 



is This and the following definitions of the family, genera, subgenera and 

 sections are based almost exclusively upon Dall's paper published in the 

 Proceedings of the Malacological Society of London, vol. 1, pp. 203-213, 1895. 



