GENERAL STRUCTURE. 



3*9 



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

 primitive form of fin known, since in an extinct armoured 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. Assuming 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 disappear- 

 ance of a considerable portion of the original fold, and 

 the development and basal contraction 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. 316. Among these the pterotic and sphenotic are 

 peculiar to the class, and there is always a large para- 

 sphenoid underlying the base of the skull. The inter- 

 vention of the elements known as the hy (/mandibular 

 and symplectic 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, inter 'opercular, and 

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

 In the gill (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 

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

 to the inner side of the hyomandibular by the stylohyal, and extends forwards 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 developed. 



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 the mouth, and even on the hyoid bones and gill-arches ; and they may 



be attached only to the membrane lining the cavity of the mouth. Frequently 



SKELETON OF PECTORAL FIN OF 

 AN EXTINCT FRINGE-FINNED 



shark. (From Fritsch.) 



