38 THE NAUTILUS. 



The criticisms we have to make upon the Synopsis of Mr. Simp- 

 son refer principally to two points: The validity of the genera 

 adopted by him, and the systematic arrangement of these groups to 

 form greater sections. Concerning the specific limits and their 

 sometimes enormous synonymy I am nearly always of the same 

 opinion with him. Some observations about, species concerning 

 which I hold divergent views, will follow at more length below. 

 I cannot say the same concerning the genera, as may easily be 

 understood. Many of the distinctive characters are derived from 

 anatomic structures, and therefore are unknown where the animal 

 has not yet been observed. The association of a number of species 

 in one genus is therefore in many cases but a provisional grouping ; 

 and specially is this the case with the Mexican and Central Amer- 

 ican species. 



The systematic arrangement of the North American Unionidce, as 

 given by Simpson in his Synopsis, is based essentially upon the 

 marsupia ; that is to say the gills or parts of them functioning as 

 ovisacs. This is a classification which we may not see emploved 

 without scruples, because it is based rather on physiologic than 

 anatomical characters. Zoological classification is founded on mor- 

 phological characters, and while physiologic or biological facts may 

 give us important explanations, they cannot be employed directly for 

 systematic purposes. This is generally acknowledged, since it be- 

 came necessary to dismember the class of Helminthes, and it is 

 likewise valid for the Mollusca, as proved by the example of the 

 pelagic gastropods. Whether the eggs expelled out of the genital 

 orifice pass into the inner gills (Endobranchia) or in the outer 

 ones (Exobranchicz) or if they fill on each side both the gills 

 (Amphib?-anchice), is a fact of merely physiologic signification, and 

 cannot be employed for systematic purposes. These differences are 

 less than those which exist in many genera or species of gastropods, 

 very closely allied, in reference to the deposition of eggs, some being 

 oviparous, others ovoviviparous or viviparous. As it seems, there 

 are in many species of the group Lampsilis and Quadrida structural 

 modifications of the outer gills, which may be used for systematic 

 purposes, but the few exact data thereon which may be found in the 

 publications of Lea, Sterki and Simpson are throughout insufficient. 

 What we need is a careful, histological study upon the gills, marsu- 

 pia and ovisacs of the North American Uniouidce. 



