PHYLUM CHORDATA 



217 



presence of a bulbus aortse, and the decussation of the optic nerves 

 indicate its position among the Teleostei. It belongs to the 



B 



FIG. 892. Hippocampus (Sea-horse). In B the opercuhnn is removed to show the gills. 

 br. a p. branchial aperture ; brd. p. brood-pouch ; d.f. dorsal fin ; g. gills ; pct.f. pectoral fin. 

 . (From Claus and Giinther.) 



Physostomi in virtue of possessing a pneumatic duct, none but 

 jointed fin-rays, and abdominal ventral fins. The characters which 

 place it among the Salmonidse are the presence of an adipose fin 

 and of pseudobranchise, the absence of oviducts, and the fact that 

 the maxilla enters into the gape of the mouth. The genus Salmo 

 is distinguished by its small scales, well-developed conical teeth, 

 absent on the pterygoids, a short ventral fin with fewer than 

 fourteen rays, numerous pyloric appendages, and comparatively 

 large ova. The distinctive characters of the various species of 

 Salmo depend upon comparatively minute points, such as the- 

 relative proportions of various parts, and are often difficult of 

 determination owing to individual variations correlated with 

 different environments. In S. fario the posterior margin of the 

 operculum is evenly curved, the maxilla is longer than the snout, 

 and the vomerine teeth are in a double series and persist throughout 

 life. 



3. GENERAL ORGANISATION. 



External Form. The typical form of the Teleostomi is very 

 fairly represented by that of the Trout (Fig. 865) a long, com- 

 pressed body, nearly half of which is formed by the tail, pointed 

 anterior and posterior ends, a large vertical tail-fin, a head of 



