34 BIOLOGICAL SOCIETY OF WASHINGTON. 



istic forms, such as families of fishes, (Cottidae, Anarrhichadidte, 

 Stichseidae, Cyclopteridse, Liparididce, Agonidae, Gadidse,) and of 

 mollusks, (Buccinidas, Lacunidse, etc. ,) as well as other invertebrates, 

 extend further southward, that the isocryme of 44° seems to be a 

 more natural approximate limit. It has the disadvantage (in whicli 

 it is not alone) of being an empirical boundary without any known 

 relationship to cause, and must be regarded simply as a provisional 

 limit. The isocryme nearly coincides with Cape Cod on the east- 

 ern, coast, and the Straits of Fuca on the western. Doubtless its 

 limits will require considerable modification hereafter, and it may 

 be doubted whether the isocryme corresponds with the natural 

 limits of the realm elsewhere. All these questions, however, must 

 be left for future investigation. 



II. THE PARARCTALIAN REALM. 



Pararctalia, or the north temperate realm, may include the various 

 coast lines between the isocrymes of 44° and 68°, the last being 

 the northern limit of the reef-growing corals. There are few, and 

 perhaps no families absolutely peculiar to it, and it is chiefly distin- 

 guished by generic and specific modifications of the Arctalian and 

 Tropicalian realms, commingling in common waters. On our east- 

 ern coast, it is characterized especially by such fish types as the 

 genera Alosa, Cyprinodon, Fundulus, Lucania, Scombresox, Siph- 

 ostoma, Menidia, Xiphias, Pomatomus, Stromateus, Roccus, Cen- 

 tropristis, Orthopristis, Stenotomus, Archosargus, Pogonias, Liosto- 

 mus, Menticirrus, Cynoscion, Tautoga, Ctenolabrus, Chretodipterus, 

 Astroscopus, Prionotus, Batrachus, Chasmodes, Hypleurochilus, Oph- 

 idium, Urophycis, Paralichthys, Pseudopleuronectes, and Malthe. 

 Not one of these belongs to a fiimily peculiar to either the special 

 region in question or the realm of which it forms a part. Most of 

 these genera are represented by congeneric species, or closely re- 

 lated generic types in other parts of the realm. 



III. THE TROPICALIAN REALM. 



Tropicalia, or the Tropical realm, may be accepted with the 



