112 



LOWER VERTEBRATES. 



All the Nematognathi are carnivorous fishes. Most of them inhabit fresh waters, 

 but many are found in the sea. The latter, however, never venture to any consider- 

 able depths, and most of them sometimes ascend the rivers. None are deep-sea 

 fishes. They are most abundant in tropical waters, a greater variety of species being 

 found in South America than in any other continent, although the rivers of Africa 

 also teem with them. Few of them reach the colder parts of the temjjerate zones. But 

 one species {Silurus glanis) is found in Europe, and none, either marine or fluviatije, 

 inhabit the waters of the United States west of the Rocky Mountains, their range in 

 the eastern Pacific not extending north of the Gulf of California. 



In size the Nematognathi vai-y greatly: from two inches in length to six feet or 

 more. Few of them are much valued as food ; most of them, especially the marine 



species, having rather tough and flavorless 

 flesh. 



As to the origin of the Nematognathi, the 

 following remarks of Professor Cojie seem to 

 me reasonable: "This division is the nearest 

 ally to the sturgeons (Chondrostei) among 

 Physostomous fishes, and I imagine that 

 future discoveries will prove that it has been 

 derived from that division by descent. . . . 

 The affinity of the cat-fishes to the sturgeons 

 is seen in the absence of symplectic, the rudi- 

 mental maxillary Ijone, and, as observed by 

 Parker, in the interclavicles. There is a sujier- 

 ficial resemblance in the dermal bones. The 

 rudimental meso]3terygium shown by Gegen- 

 baur to exist in the young Siluridie, the prae- 

 coracoid arch, and the ventral fins, are shared 

 with the sturgeons and other divisions." 



One of the families (Hypophthalmidai) 

 at jiresent referred to this order, differs from 

 all others, according to Gq\iq, in having the 

 lower pharyngeals united for tlieir whole 

 length. 



The Nematognathi are all placed by Dr. 

 Giinther in a single family, Siluridie. This 

 is certainly unnatural, as the differences between many of the different members 

 of the group far outweigh the distinctions existing between the currently recognized 

 families of Acanthopteri. Professor Cope divides them into three families, Silurida;, 

 AspredinidsB, and Hypophthalmidaj. Professor Gill arranges them in eleven families. 

 The jH-esent writer has had no opportunity to form a definite opinion of the value of 

 most of these families, but for the present is disposed to follow Professor Gill's 

 arrangement. The different families now follow in order. 



Tlie family of Aspkedinid^ is characterized especially, according to Cope, by the 

 absence of the opercle. The adipose fin is wanting, and there is no sjiine in the dorsal. 

 The eyes and mouth are small. There are six barbels. The gill o]iening is reduced 

 to a small foramen. The skin is either smooth or covered with bony lumps or 

 tubercles. About ten species are known, all from the fresh waters of the north- 



