OSTARIOPHVSI 587 



on the maxillary bones (Diplomystes, ^utrojnichtJujs), being 

 usually confined to the praemaxillaries and dentaries ; they often 

 occur on the palate. The branch iostegal rays vary from 4 to 

 17. The lips are sometimes much developed, and may form a 

 sucking disk, as in Eucliilichthys and Exostoma. As in the 

 Cyprinids, the pungent spines which may arm the fins have 

 nothing in common with tlie true spines of Acanthopterygians ; 

 they result from the co-ossification, with age, of successive articles ; 

 but, contrary to the condition in the Cyprinids, the axis of the spine 

 is single, not double. The ventral rays vary from 6 to 1 6, 6 being 

 the most frequent number. Some of the exterior vertebrae may 

 be solidly fused together, and also with the occipito- nuchal 

 buckler. Prof Eamsay "Wright^ has shown, by a study of the 

 development, that the complex which follows the first vertebra, 

 which is more or less rudimentary, if distinct, represents the 

 fusion of the 2nd, 3rd, and 4th vertebrae, without even nerve- 

 foramina denoting its compound origin ; the first, strongly 

 developed, transverse process represents that of the 4th vertebra. 

 The air-bladder is usually large and trilocular, but additional 

 septa may greatly complicate its structure, and external diverticula 

 may also exist f it may be more or less reduced and entirely or 

 partially enclosed in a bilateral bony capsule formed by the 

 transverse processes of the vertebrae, and sometimes {Nemato- 

 genys) ankylosed to the skull. In a few genera, like Cetoj^sis, the 

 air-bladder seems to be altogether absent : it is reduced to two 

 small oval sacs encased in the large compound anterior vertebra. 

 As in Loaches, the air-bladder is often in immediate contact with 

 the skin behind the shoulder-girdle. The intestinal tract may 

 be simple and short (carnivorous forms) or extremely long and 

 convoluted {Ccdliclithys) ; as in Cyprinids, pyloric appendages are 

 absent. 



Cat-Fishes, as Silurids are usually called, are a large family 

 embracing some 1000 species, spread over the freshwaters of all 

 parts of the world, but mostly from between the tropics.^ Only 

 a few are marine {Plotosus, Arius, Galeichthys). 



This family may be divided into eight sub-families 



' Proe. Gmuul. Inst. (2) ii. 1884, p. 376. 



"- Of. Bridge and Haddon, Phil. Trans. R. Soc. clxxxiv. 1893, p. 65. 



' The absence of these fishes from the United States west of the Rocky Mountains 

 is very remarkable. Amiurus nebulosus was introduced about 1877 into some parts 

 of California, where it is said to be now excessively abundant. 



