958 



CHAPTER XLIX. 



CLASS PISCES — continued. 



Order Ganoidei. 



Order V. Ganoidei. — The Ganoids form an order exceedingly 

 difficult of definition, owing to their close connection on the one 

 hand with the Dipnoi and (through the Acanthodea) the Elasmo- 

 branchei, and on the other with the Teleostei, and it is not improb- 

 able that it may be eventually necessary to divide them into at least 

 two orders. As mentioned above, some writers group them in a 

 subclass with the Teleostei under the name of Teleotomi. The 

 body may be either naked or covered with shagreen skin, or with 

 large detached bony scales, or completely covered either with true 

 ganoid scales, or with cycloidal scales of a ganoid structure. The 

 vertebral column, again, may be either cartilaginous or fully ossified ; 

 and its termination in the tail may be either diphycercal or hetero- 

 cercal. Paired and median fins are generally present, and the pelvic 

 pair (with perhaps one exception) is abdominal in position. The 

 skull may either be covered merely by cranial membrane bones, or 

 may be completely ossified. It is hyostylic — i.e., there is a hyoman- 

 dibular suspensorium — and the palatopterygoid is distinct from the 

 cranium ; and, as a general rule, even in the cartilaginous forms 

 maxillse and dentary bones, which carry the teeth, are developed on 

 the palatopterygoid and Meckel's cartilage respectively. The gills 

 are usually free, and their single aperture is covered by an oper- 

 culum ; while branchiostegal rays are very generally present. In 

 most cases a secondary pectoral girdle of dermal bones (clavicular, 

 supraclavicular, &c.) is developed externally to the cartilaginous 

 scapulo-coracoidal girdle, which alone exists in the preceding orders. 

 Finally, there is a swim-bladder, with a duct into the pharynx • while 

 there are some other characters of the soft parts into the considera- 

 tion of which it will not be necessary to enter in this work. 



Some very curious features occur in the ossification of the ver- 



