CLASS PISCES. 587 



hollows, and foramina, certain regions analogous to 

 those of the cranium of other fishes. It will even 

 happen that some mobile articulations, existent in the 

 other orders, do not manifest themselves at all in this : 

 for example, part of the vertebrae of certain rays are 

 united into a single body ; some of the articulations 

 of the bones of the face also disappear, and even the 

 most apparent character of this division of the class 

 of fishes is to want the maxillary and intermaxillary 

 bones, or rather to have them only in vestiges con- 

 cealed under the skin, while their functions are per- 

 formed by bones analogous to the palatines, and 

 sometimes even by the vomer. The gelatinous sub- 

 stance which, in the other fishes, fills the intervals of 

 the vertebrae, and communicates from one to the 

 other only by a small hole, forms, in several chon- 

 dropterygii, a cord which runs through all the bodies 

 of the vertebrae, almost without varying in diameter. 

 This series is divided into two orders : the chon- 

 dropterygii, whose gills are free, like those of 

 ordinary fishes, and those whose gills are fixed, that 

 is, attached to the skin by their external edge, so that 

 the water does not issue from their intervals, except 

 through the holes of the surface. 



