I. Cyclostomi II. Selachii III. Teleostomi, the characters and contents of which will be found in separate articles; in the present state of uncertainty as to their position, _Palaeospondylus_ and the _Ostracodermi_ are best placed _hors cadre_ and will be dealt with under these names. The three subclasses here adopted correspond exactly with those proposed in Theo. Gill's classification of the recent fishes ("Families and Subfamilies of Fishes," _Mem. Nat. Ac. Sci._ vi. 1893), except that they are regarded by that authority as classes. The period dealt with in this chapter, ushered in by the publication of Günther's _Introduction to the Study of Fishes_, has been one of extraordinary activity in every branch of ichthyology, recent and fossil. A glance at the _Zoological Record_, published by the Zoological Society of London, will show the ever-increasing number of monographs, morphological papers and systematic contributions, which appear year after year. The number of new genera and species which are being proposed is amazing, but it is difficult to tell how many of them will simply go to swell the already overburdened synonymy. Perhaps a reasonable estimate of the living species known at the present day would assess their number at about 13,000. It is much to be regretted that there is not a single general modern systematic work on fishes. The most important treatises, the 7th volume of the _Cambridge Natural History_, by T. W. Bridge and G. A. Boulenger, and D. S. Jordan's _Guide to the Study of Fishes_, only profess to give definitions of the families with enumerations of the principal genera. Günther's _Catalogue of the Fishes in the British Museum_ therefore remains the only general descriptive treatise, but its last volume dates from 1870, and the work is practically obsolete. A second edition of it was begun in 1894, but only one volume, by Boulenger, has appeared, and the subject is so vast that it seems doubtful now whether any one will ever have the time and energy to repeat Günther's achievement. The fish fauna of the different parts of the world will have to be dealt with separately, and it is in this direction that descriptive ichthyology is most likely to progress. North America, the fishes of which were imperfectly known in 1880, now possesses a _Descriptive Catalogue_ in 4 stout volumes, by D. S. Jordan and B. W. Evermann, replacing the synopsis brought out in 1882 by D. S. Jordan and C. H. Gilbert. A similar treatise should embrace all the fresh-water species of Africa, the fishes of the two principal river systems, the Nile and the Congo, having recently been worked out by G. A. Boulenger. Japanese ichthyology has been taken in hand by D. S. Jordan and his pupils.