18 BIOLOGICAL SOCIETY OF WASHINGTON. 



fresh-water fishes, and i family and 6 or 7 species of Marsipobran- 

 chiates. Of these, several are wholly or almost peculiar to the re- 

 gion. These are, of mammals, the Antilocapridae, Zapodidte, 

 Gcomyidce, and Haploodontid^ ; of birds, the Chamcxidte (of very 

 doubtful value) ; of reptiles, the LichanuridK ; of amphibians, (ac- 

 cording to Cope,) the Scaphiopodidte, Plethodontida, Amblysto- 

 mid^, Amphiumidaj, and Sirenidte ; and of fishes, the Centrar- 

 chid«, Elassomidae, Aphredoderidre, Amblyopsidce, Percopsidae, 

 Hyodontidae, and Amiid^e. Of the others, those characteristic of 

 the northern portions of the region are shared in common with 

 Europe and Northern Asia, while such as are especially represented 

 in the southern portions are held in common with South America. 

 On the whole, however, the predominant type of the region — and 

 only more decidedly so to the northward — is akin to that of the 

 Eastern hemisphere, and the South American aspect given to the 

 region is apparently due rather to the (geologically) recent intru- 

 sion of South American types than to its being the natal abode of 

 such forms. 



Of the other classes of animals, those most noteworthy are the 

 gastropods and conchifers ; the former is extremely rich in species 

 of the family of CeriphasiidcC, which includes Melanioids of peculiar 

 genera; the latter is even to a still greater degree exuberant in 

 species representing also numerous sections, or sub-genera, peculiar 

 to the region. In fact, over 400 species of gastropods, most of 

 which belong to the Ceriphasiidse, and about 600 species of con- 

 chifers, the greater portion of which are members of the family of 

 Unionidce, have been attributed to the region, but these numbers 

 are undoubtedly greatly exaggerated. It may be added, in con- 

 clusion, that several of the families and genera now peculiar to the 

 realm were, in early geological epochs, represented elsewhere, the 

 Lepidosteidae, for instance, having formerly had members in Eu- 

 rope. Several of the peculiar genera are also of great antiquity, 

 characteristic sections or genera of Viviparid?e, Melaniidse, and 



