FISHES. 309 



case, modelled besides, and pierced very nearly as the cranium is in 

 the common fish, so that this head of the chondroptcrygians is capable 

 of being separated into the same regions, the same fossae, the same 

 eminences;, and the same foramina, but is not composed of bones 

 so capable of being separated. 



Their face is also extremely simple. There are only two liones to 

 their j)alato- temporal arch, the first jjroceeding downwards from the 

 cranium to the articulation of the jaws, the second holding the place 

 of the superior jaw, and supporting the teeth, whilst the maxillary and 

 the intermaxillary are degenerated to very trifling traces which are 

 concealed in the thickness of the lip. Neither has the lower jaw 

 more than one bone also on each side (the articular) w])ich supports 

 the teeth, and there only remains of the others a slight vestige which 

 is also hidden beneath the skin of the lip. 



The opercular apparatus is; wanted ; but that of the hyoid and the 

 branchiee has a good many characters in common with the same appa- 

 ratus in the osseous fishes. Besides, there is in the sea-dogs, opposite 

 the external attachment of each branchia, a thin bone suspended 

 beneath the integuments, which is the true rudiment of a rib, but is 

 very different from the branchiostegal rays that are considered in the 

 osseous fishes as sternal ribs. The branchial apparatus is placed far- 

 ther behind in the chondropterygians than it is in the osseous fishes, 

 and it is that which so much draws back, beneath the origin of the 

 spine, the girdle of the shoulder. 



The latter which is attached, in the rays, exclusively to large 

 apophyses of the sphine, but which is free from any adhesion in the 

 squalus is a single piece only, in these two genera, and it surrovmds 

 the body, and supports on each side a greater or less numerous roAV of 

 other pieces, which form a base for the pectoral fin, and also for the 

 borders to which the rays adhere. 



The pelvis, in the same way, is only a single transverse piece, 

 which does not articulate with the spine, and supports, on each side, a 

 lamina or stem to which the rays of the ventral adhere. This is the 

 stem which is continued in the form of a mass in the males, and in 

 them assumes a very complicated sort of structure, respecting which 

 we shall have more to say, liy and bye. 



There are portions of the spine in which several of the vertebrde 

 are soldered together, or, at least, in which the space where they are 

 to be laid is occupied only by a tube of a single piece, pierced on each 

 side by numerous foramina for the exit of so many pair of nerves, 

 Such is the origin of that of the rays. In the latter also, and in the 

 squalus we may observe, that there are just twice as many superior 

 rings as there arc vertebrjie. Independently of the common annular 

 I)ortions, there are others wJiich correspond to the joinings of the 

 vertebrae altogether. 



The spinal ribs, whenever they exist in fishes, are usually very 

 small, and in the rays, these ribs are much more so than those of the 

 squalus. The sturgeons have very considerable spinal ribs. 



In respect of the structure of the branchiae, the sturgeon is a mediiun 

 betAveen the two genera just mentioned, and the common fishes ; 

 many of the bones of the head, and all the bones of the shoulder being 



