914 



CLASS PISCES. 



pectoral fins may be developed into huge dermal spines, which in 

 some cases articulate by a complete shackle-joint with the basal 

 bones. Similar spines are also developed in the Elasmobranchei 



Fig. 83 

 toral 



37. — Left lateral view of the Perch (Perca). o, Gill-cover, with gill-slit behind if, p, Pec- 

 fin; v, Pelvic do. ; d, First dorsal do. ; d' , Second dorsal do. ; /, Lateral line. Reduced. 



(fig. 838); but here they are simply inserted into the flesh, or are 

 attached only by cartilage, so that their basal end is rounded off. 

 Similar spines (fig. 838, 1) may also occur behind the head. Such 



. Fig. 838.— Spikes and teeth of Elasmobranchei. i, Nuchal spine of Pleuracanthus ; 2, Fin- 

 spine of Gyracanthus ; 3, Do. of Ctenacanthus ; 4, Tooth of Petalodus ; 5, Do. of Psavnnodus; 

 6, Do. of Ctenoptychius. Carboniferous. 1-3 are reduced. 



spines when found fossil are frequently known as ichthyodorulites, 

 and form important objects to the palaeontologist. Their structure 

 is identical with that of the so-called placoid scales and of teeth, 



