XXI OSTARIOPHYSI 587 
on the maxillary bones (Diplomystes, Hutropiichthys), being 
usually confined to the praemaxillaries and dentaries ; they often 
occur on the palate. The branchiostegal rays vary from 4 to 
17. The lips are sometimes much developed, and may form a 
sucking disk, as in Huchilichthys and Exzostoma. As in the 
Cyprinids, the pungent spines which may arm the fins have 
nothing in common with the 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 16, 6 being 
the most frequent number. Some of the exterior vertebrae may 
be solidly fused together, and also with the occipito-nuchal 
buckler. Prof. Ramsay 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 ;* 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 (em«ato- 
genys) ankylosed to the skull. In a few genera, like Cetopsis, 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 sumple and short (carnivorous forms) or extremely long and 
convoluted (Callichthys) ; a8 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 
1 Proc. Canad, Inst. (2) ii. 1884, p. 376. 
2 Cf. Bridge and Haddon, Phil. Trans. R. Soc, clxxxiv. 1893, p. 65. 
3 The absence of these fishes from the United States west of the Rocky Mountains 
is very remarkable. Aimiwrus nebulosus was introduced about 1877 into some parts 
of California, where it is said to be now excessively abundant. 
