OSSEOUS SYSTEM. 197 



edge the digital portion or abdominal fin ; it is rare for a third set of 

 bones to be interposed between these. 



More diversities appear in the posterior than in the anterior ex- 

 tremities ; thus they are often completely absent, as in Xiphias and 

 Fishes of the Eel kind, e.g., Muraena, Mursenophis, Sphagebranchus, 

 Gymnotus, &c., and are placed in some instances far forward, in 

 others far backward ; upon these varieties of position of the ventral 

 fins the old Linnsean division of Fishes into Apodal, Abdominal, Pec- 

 toral, and Jugular, was based. 



Both pelvic or iliac bones are usually more or less united to each 

 other ; in some Fishes they are completely anchylosed, in others 

 prolonged backward in the form of a spine, as in Cyprinus, Scomber, 

 Zeus, &c. ; they are occasionally separate, as in Lophius, Batrachus. 

 Polypterus possesses four elongated bones, that correspond as the 

 third division of the extremities to the tarsus and metatarsus. 



The Skeleton in the Cartilaginous differs altogether in structure 

 from that already described as belonging to the Osseous Fishes, and 

 it is only its fundamental types which are common to the two as 

 Vertebrate animals. It is scarcely possible to offer any general 

 remarks relative to the skeleton of the Cartilaginous Fishes, and we 

 must therefore characterize its peculiarities in the several orders and 

 families. 



In the Cartilaginous Fishes the skeleton remains throughout life 

 in a cartilaginous condition, and true ossification occurs only in the 

 tegumentary bones, e. g., in the Sturgeon, or in particular parts of 

 the skeleton. In some Cartilaginous Fishes, such as those with free 

 branchiae (Acipenser, Chimaera), the permanent cartilage resembles 

 the cartilage that precedes the formation of bone in the Osseous 

 Fishes : the cyclostomi, however, are distinguished by possessing 

 a very peculiar kind of cartilaginous tissue with coarse cells, while 

 the Plagiostomi possess the cartilaginous tissue of the Sturgeon and 

 Chimera ; and in addition thereto, other kinds of cartilaginous tissue, 

 namely, 1st. The hyaline or transparent tissue, with more or less 

 membranous cartilaginous corpuscules ; this occurs in the Sturgeon, 

 and also in the Osseous Fishes, and forms mostly the internal part 

 of the cartilage, with the exception, however, of the vertebral bodies. 

 *2d. The tessellated calciferous cartilaginous tissue, which occurs 

 only in the Sharks and Rays, and invests the hyaline cartilage like 

 a hard crust, with the exception of the vertebral bodies. This car- 

 tilage consists of prismatic pieces which resemble brick-work, and 

 are separable from each other. 3d. The cellular cartilaginous tissue 



