CHAPTER ni. 



THE IBOVINCES OF THE VERTEBHATA THE CLASS PISCES. 



The Vertebrata are divisible into three primary groups or 

 provinces : the Ichthyopsida, the Saurqpsida, and the Mam- 

 malia. 



I. — The Ichthyopsida 



1. Have the epidermic exoskeleton either absent, or very 

 slightly represented. 



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

 membranous 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 parasphenoid. 

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



4. 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 un- 

 ossified or very imperfectly ossified. 



5. The mandible may be absent, or be represented onlj' by 

 cartilage. If membrane bones are developed in connection 

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

 articular element may be ossified or not, and may be con- 

 nected with the skull by the intermediation of a quadrate and 

 a liyomandibular element, or by a single fixed plate of carti 

 lage representing both these and the pterygo-palatine arch. 

 A stapes may be present or absent. 



6. The alimentary canal may or may not terminate in a 

 cloaca. When there is no cloaca, the rectum opens in front of 

 the urinary organs. 



7. The blood-corpuscles are always nucleated, and the 

 heart mav be tubular, bilocular, or trilocular. 



