GENERAL CHARACTERISTICS 



2687 



the nature of clumsy paddles, and being adapted for slowly-moving forms like the 

 lungfishes, which pass most of their time among the mud at the bottom of rivers. 



It must not be supposed, however, that even the fringed type is the most primi- 

 tive form of fin known, since in an extinct armored shark {Cladoselache} from the 

 Carboniferous formation, we meet with what may be called the fold type of paired 

 fins. In these fishes (one of which is figured in the sequel) the pectoral and pelvic 

 fins are placed far apart, but in the same longitudinal line, and are formed of a 

 series of parallel cartilaginous rods arising from an extended base, and projecting 

 at right angles to the body; the pectoral pair being considerably the larger. Assum- 

 ing that fishes originally possessed on each side of the body a continuous fold of 

 skin, strengthened by parallel cartilaginous rods projecting at right angles, this 

 fold type is exactly what we should expect to find in the evolution of pectoral and 

 pelvic fins, by the disappearance of a considerable portion 

 of the original fold, and the development and basal con- 

 traction of the remaining moieties. 



Although the structure of the skull is of the highest 

 importance in the classification of fishes, our remarks on 

 this subject must be very brief. In the skulls of the 

 higher bony fishes the original cartilaginous cranium, 

 which persists in the sharks, is overlain by a number of 

 membrane bones, the names and position of which are 

 indicated in the figure of the perch's skeleton on p. 2684. 

 Among these the pterotic and sphenotic are peculiar to the 

 class, and there is always a large parasphenoid underly- 

 ing the base of the skull. The intervention of the ele- 

 ments known as the /lyomandtbular and symp/ectic between 

 the squamosal and the quadrate is unknown elsewhere, 

 although it is by no means universal among fishes. 

 Among other bones may be mentioned the infraorbital 

 ring beneath the eye; as well as the preopercular, subopercular , interopercular ; and 

 opercular, collectively constituting the gill cover or operculum of the bony fishes. 

 In the7// (or branchiostegal} membrane which joins with the gill cover in closing in 

 the gill chamber, there may be developed a number of gill (or branchiostegal) rays; 

 but these may be partially or entirely replaced by jugular plates, occupying the 

 space between the two branches of the lower jaw. It will be unnecessary to men- 

 tion by name the various bones constituting the hyoid arch, which is attached to the 

 inner side of the hyomandibular by the stylohyal, and extends forward to support 

 the tongue; and it will suffice to state that behind this arch are situated the branchial 

 or gill arches, to the inner margins of which are attached the spine-like gill rakers. 

 In the lower jaw or mandible, there is usually both a dentary and an articular piece; 

 but an angular, and more rarely a splenial or coronoid element may likewise be de- 

 veloped. 



T ., The teeth of fishes present a greater degree of variation than is 



found among any other class of Vertebrates. While in some cases 



they may be totally wanting, in others they may be developed on all the bones of 



SKELETON OF PECTORAI, FIN 

 OF AN EXTINCT FRINGE- 

 FINNED SHARK. 

 (From Fritsch.) 



