PISCES. 61 



The cranium in the simplest cases (e.g. Selachii) forms a 

 cartilaginous box enclosing the brain and sense organs ; in 

 bony fishes it is greatly complicated. When palatine or 

 pterygoid bones are present they are formed by the ossifi- 

 cation of cartilage ; in Sauropsida and Mammalia they are 

 laid down as membrane bones. There is no tympanic cavity 

 or auditory ossicle in relation to the ear. 



There are two principal types of suspensorium by means of 

 which the jaws are attached to the cranium : 



(1) The Autostylic. This is the primitive condition 

 in which the mandibular arch articulates with the base of the 

 cranium in front of the hyoid and in a similar manner. 



(2) The Hyostylic. In this case the mandibular arch 

 becomes connected with the hyomandibular and supported by 

 the hyoid arch. These terms are more fully discussed in 

 Chapter VIII. 



There is always an internal framework supporting the 

 gills ; it usually consists of the hyoid arch and five, rarely 

 six or seven, pairs of branchial arches. The limbs are repre- 

 sented by two pairs of fins, the pectoral and the pelvic ; they 

 are not divided into proximal, middle and distal portions. The 

 ribs do not unite with a median ventral sternum, or meet in 

 the midventral line in any other way in the trunk region. 



Order I. ELASMOBRANCHII. 



The exoskeleton is in the form of placoid scales which are 

 sometimes so numerous as to give the whole skin a rough 

 surface forming shagreen. In some cases the placoid scales 

 are enlarged to form plates or spines capped or coated with 

 enamel. These spines may be imbedded in the flesh in front 

 of the paired or unpaired fins, or may be attached to the tail. 

 They are specially characteristic of the suborder Acanthodii. 

 The endoskeleton is cartilaginous and true bone is never 

 found. Much of the skeleton, especially of the vertebral 



