THE FAMILY OF NAIADES. 115 



them ; on the contrary, they pass, by means of various intermediate 

 species, into the other genera, tribes, orders, &c., to which they are 

 most nearly allied. For this reason, viz., that no fixed limits can be 

 assigned to orders, genera, &c., we find the ideas about them fluctuat- 

 ing with the degree of our knowledge ; which is the true cause of those 

 changes in the limits of genera, &c., which persons unacquainted with 

 the subject are apt to consider arbitrary ; but which, in skilful hands, 

 are dependent upon a progressive advance in the knowledge of science." 



Blainville, in his " Manuel de Malacologie," divides the Naiades (his 

 Sub-Mytilacea) into Anodonta and Unio^ but thinks that species will 

 be found which will make these to be united.* 



Sowerby says, " the difficulty of ascertaining to which genus of La- 

 marckian Naiades certain species belong, arises from the very general 

 similarity of form," &c. ; "in fact, an examination of a sufficient num- 

 ber of species will prove that no dependence can be placed upon the 

 characters by which authors usually attempt to discriminate between 

 these genera, and that the transition from one to another is so gradual 

 in some instances, and so strongly marked in others, that it is not sur- 

 prising that authors who having only met with certain species, and not 

 being aware of such intermediate links, should have considered them 

 as the types of new genera."t And further, " we think we have al- 

 ready said enough to prove, that unless it be thought wise to elevate 

 each of the peculiar sorts we have mentioned, and many more, into dis- 

 tinct genera, it will be positively necessary to unite them altogether 

 under one generic appellation." Swainson (Zool. Illus., second series) 

 divides this family into Unio, Hyria, Iridina, ^nodon, and Alasmodon, 

 but in describing An. areolatus speaks of the genera gliding into each 

 other. 



Deshayes, in his edition of Lamarck's "Animaux sans Yertebres," 

 says it is impossible to separate the genera of the Naiades. " Nous 

 pourrions prendre pour exemple celui des genres qui est considere 

 comme I'un des mieux caracterises. Le genre Sympkynote est fonde 

 sur ce caractere remarkable que les deux valves sont soudees entre elles 

 le long du bord superieur," etc. "Nous concluons que tout ce grand 



* See page 540, t Zool. Journ. Vol. I. 



