WHITE ON PALEONTOLOGY. 



619 



The genus Anodonta is notably free from any marked differentiation 

 into subgenera or sections, similar to those of Unio, although the 

 species are somewhat numerous and common to all parts of the world. 

 The genus 21argaritana is much more differentiated than Anodonta,, but 

 very much less so than Unio. I shall therefore consider only the sec- 

 tions or subordinate types of Unio in the remainder of this paper. 



Dr. Lea has so tabulated the species in bis Synopsis of the Family 

 Unionidre as to indicate the marginal outline and surface-features of 

 'each. As it is very largely by these characters that the types before 

 referred to are expressed, I give a list of the terms he has used to in- 

 dicate the outline-form and character of surface, for the purpose of 

 using them iu making- comparisons of our own forms with those of 

 other parts of the ^orld: — 



Form. Surface. 



Quadrate. Smooth. 



Triangular. Plicate. 



Oblique. Nodulous. 



Oval. Sulcate. 



Oblong. Spinous.- 



Subrotuud. 

 Wide, 

 Obovate. 

 Arcuate. 



It is the oval form and the smooth or plain surface that are found to 

 be more common than all other features, and to prevail in all parts of 

 the world ; while the oblique, quadrate, and subrotuud forms, and 

 nodulous and spinous surfaces are more restricted than any of the 

 others. It is these restricted features that are among the leading char- 

 acteristics of our Xorth American types of Unio. 



The following partial statements, mostly collated from Dr. Lea's 

 Synopsis, illustrate in part the continental restrictions just referred to, 

 the figures indicating the number of species iu each case : — 



FOUM. 



Oblique 



Submtnud 



Quadrate 



SLT.FACE. 



Smooth 



Plicate 



Nodnloua 



Sulcate 



Spinous 



:jirorth 

 America. 



500 

 •2"J 

 43 

 13 



South and 

 Central 

 America. 



Europe. 



All 



It is not claimed that the marginal outline as given by Dr. Lea in- 

 variably indicates a definite type or section of the genus, for it does so 

 only approximately ; but this method of presenting the subject seems to 

 be the best that is at present available. For example, the "wide" 



