PISCES FISHES. 495 



cranial bones of higher animals are composed, remain here per- 

 manently separated, overlapping each other so as to form squamous 

 sutures ; but never becoming fused together, as the elements of the 

 human skull invariably do at a very early period. 



No fewer than twenty-six bones enter into the composition of 

 the cranium we are now considering ; to which, as is now generally 

 allowed, the following names are applicable. 



The Frontal bones are each divided into three portions, called 

 respectively the Principal frontal (1),* the Anterior frontal (2), 

 and the Posterior frontal (4). 



Between the anterior frontal bones is the Ethmoid, a simple 

 vertical lamella, which is often merely a cartilaginous plate. 



The middle of the base of the cranium is made up of two bones : 

 the Basitar (Jig. %$. 5), a portion of the occipital forming the 

 body of the occipital vert'ebra ; and the body of the Sphenoid (6), 

 a distinct bone, which is prolonged anteriorly into a lengthened 

 process, which serves as the base of the membranous septum be- 

 tween the orbits. 



The Parietal bones (7) are placed behind the posterior frontal, 

 but they do not generally touch each other, being separated by an 

 interposed bone called the Interparietal (8) . 



The Occipital bone is made up of five portions, namely, two 

 External Occipitals (9), two Lateral Occipitals (10), and the 

 Basilar bone (5), already noticed, by which the head is articulated 

 with the first vertebra of the spine. 



Two detached bones, which represent the great or temporal ala 

 of the Sphenoid, fill up the space between the body of the Sphe- 

 noid and the posterior frontal. 



Two other pairs of bones, which are elements of the temporal 

 bone in man, likewise assist in forming the cranium : these are 

 called the M astoid bones (12), and the Petrous bones (13). 



A single bone, analogous to the anterior portion of the body 

 of the human Sphenoid, and which, as will be fully evident here- 

 after, is essentially distinct from the posterior portion, bears the 

 name of the Anterior Sphenoid, while the orbital ala of the 

 Sphenoid are found in the two bones marked 14. 



These, therefore, together with the representative of the Vomer 

 (16), complete the cranial portion of the skull ; no fewer than six 

 azygos and twenty pairs of bones entering into its composition. 



* In order lo simplify the subject as much as possible, and prevent unnecessary 

 repetition, the reader will observe that, throughout all the figures connected with the 

 osteology of the Vertebrata, corresponding bones are indicated by the same numbers. 



