112 THE ANATOMY OF VERTEBEATED ANIMALS. 



CHAPTER III. 



THE PROVINCES OF THE VERTEBRATA — THE CLASS PISCES. 



The Vertebrata are divisible into three primary groups or 

 provinces : the Iclithyopsida, tlie Sauro]psida, and the Mam- 

 malia. 



I. — The Ichthyopsida 



1. Have the epidermic esoskeleton either absent, or very 

 slightly represented. 



2. The spinal column may persist as a notochord with a 

 meml^ranous sheath, or it may exhibit various degrees of 

 chondrification or ossification. When the vertebrae are dis- 

 tinct their centra have no epiphyses. 



3. The skull may be incomplete and membranous, more 

 or less cartilaginous, or osseous. When membrane bones 

 are developed in connection with it, there is a large pai'a- 

 sphenoid. The basisphenoid is always small, if it be not 

 absent. 



•i. The occipital condyle may be absent, or single, or 

 double. When there are two occipital condyles they belong 

 to the ex-occipital region, and the basi-occipital region is 

 uuossified or very imperfectly ossified. 



5. The mandible may be absent, or be represented only 

 by cartilage. If membrane bones are developed in con- 

 nection with it, there is usually more than one on each side. 

 The articular element may be ossified or not, and may be 

 connected with the skull by the intermediation of a quadrate 

 and a hyomandibular element, or by a single fixed plate of 

 cartilage representing both these and the pterygo-palatine 

 arch. A stapes may be present or absent. 



