2. The “Philosophia Ichthyologica” is devoted to a description of the external and internal parts of fishes; Artedi fixes a precise terminology of all the various modifications of the organs, distinguishes between those characters which determine a genus and such as indicate a species or merely a variety; in fact he establishes the method and principles which subsequently have guided every systematic ichthyologist. 3. The “Genera Piscium” contains well-defined diagnoses of forty-five genera, for which he fixes an unchangeable nomenclature. 4. In the “Species Piscium” descriptions of seventy-two species, examined by himself, are given; descriptions which even now are models of exactitude and method. 5. Finally, in the “Synonymia Piscium” references to all previous authors are arranged for every species, very much in the same manner which is adopted in the systematic works of the present day. [Sidenote: Linnæus.] Artedi has been justly called the Father of Ichthyology. So perfect was his treatment of the subject, that even LINNÆUS could no more improve it, only modify and add to it; and as far as Ichthyology is concerned, Linnæus has scarcely done anything beyond applying binominal terms to the species properly described and classified by Artedi. Artedi had divided the fishes proper into four orders, viz. _Malacopterygii_, _Acanthopterygii_, _Branchiostegi_, and _Chondropterygii_, of which the third only, according to our present knowledge, appears to be singularly heterogeneous, as it comprises _Balistes_, _Ostracion_, _Cyclopterus_, and _Lophius_. Linnæus, besides separating the Cetaceans entirely from the class of fishes (at least since the 10th edition of the “Systema Naturæ”) abandoned Artedi’s order of Branchiostegi, but substituted a scarcely more natural combination by joining it with Artedi’s Chondropterygians, under the name of “Amphibia nantes.” His classification of the genera appears in the 12th edition of the “Systema,” thus-- AMPHIBIA NANTES. _Spiraculis compositis._ Petromyzon. Raia. Squalus. Chimæra. _Spiraculis solitariis._ Lophius. Acipenser. Cyclopterus. Balistes. Ostracion. Tetrodon. Diodon. Centriscus. Syngnathus. Pegasus. PISCES APODES. Muræna. Gymnotus. Trichiurus. Anarhichas. Ammodytes. Ophidium. Stromateus. Xiphias. PISCES JUGULARES. Callionymus. Uranoscopus. Trachinus. Gadus. Blennius. PISCES THORACICI. Cepola. Echeneis. Coryphæna. Gobius. Cottus. Scorpæna. Zeus. Pleuronectes. Chæetodon. Sparus. Labrus. Sciæna. Perca. Gasterosteus. Scomber. Mullus. Trigla. PISCES ABDOMINALES.