HANNIBAL: CALIFORNIAN FRESHWATER MOLLUSCA. 119 



derived from the same immediate stock constitutes a superspecies or 

 ' group ' of species. Similarly, a genus is a group of allied species 

 evolving along approximately parallel lines, and a family a group of 

 genera whose history is essentially analogous, while sub-genera and 

 sub-families, and supergenera or ' groups ' of genera and superfamilies, 

 bear analogous relations to genera and families respectively that sub- 

 species and superspecies do to species. 



Applying these criteria to the present group it is obvious that as 

 a whole the jS'aiades constitute a well-detined superfamily, the 

 Union oidese, while the different development stages, of which there 

 are nine, constitute sub-families whose relations may be graphically 

 expressed by the accompanying diagram. These belong to five 

 radicles, and these radicles the writer would regard as families since 

 they are more or less equal in value and conform to the definition. 

 Tlierefore, on the basis of our present knowledge the Naiades may be 

 tentatively classified as follows : — 



Superfamily UNIONOIDE^ (Swainson), 1840. 



Margaritanid^ (Ortmann), 1910. 



Hyriid^ (Swainson), 1840. 

 Ilyriince, s.s. 

 Midelinm (Gray), 1847. 



Quadrueid,e (Von Ihering), 1901. 

 QuadndincB, s.s. 

 Fleurohenmice, nov. 



Lampsilid^ (Von Ihering), 1901. 

 ZampsiliiKC, s.s. 

 Propterince, nov. 



Unioxid^, Swainson, 1840. 

 Unio7iince, s.s. 

 A)iodotitin(S, Swainson, 1840. 



It is probable that with a knowledge of more genera than have at 

 present been studied and made known, ^ the number of families and 

 sub-families will be considerably increased since the internal characters 

 of this group have specialized along a series of similar but distinct lines. 



Family MAKGARITAKID^ (Ortmann), 1910. 



Shell of moderately large size, quadruliforra, unioniform, or 

 margaritaniform ; animal as described above, tachytictic ; glochidium 

 simple ; habitat fluviatile and lacustrine. 



The Margaritanidse, in spite of the fact that modifications of tlie 

 shell have taken place to a very considerable degree in all the known 



1 See A. E. Ortmann, Nautilus, xxiii, pp. 114 ff., 139 ff., 1910 ; xxiv, pp. 39 ff., 

 94 ff., 1910; pp. 114 ff., 1911; xxv, pp. 5 ff., 1911 ; Mem. Cam. Mus., 

 iv, pp. 279-347, pis. Ixxxvi-ix, 1911 ; Lefevre & Curtis, Journ. Exp. ZooL, 

 ix, pp. 79-115, pis. i-v, 1910. 



