52 MODIFICATIONS OF THE JAWS OF FISHES. 



The term " cranium" might well be applied to the four 

 neural arches collectively, but would exclude some bones 

 called "cranial," and include some called "facial," in Hu- 

 man Anatomy. In a side view of the naturally-connected 

 bones of the head of a fish, such as is shown in the figure 

 of the skeleton of the sea-perch. Cut 9, the upper part of 

 the head is formed by the neural spines called sqperocci- 

 pital, 8, frontal, 11, and nasal, 15; produced at the hinder 

 half into the median ridge. The right lateral ridge is 

 formed by the parietal, 7, and paroccipital, 4; the external 

 ridge by the post-frontal, 12, and the mastoid, 8. The 

 anterior termination of the series of centrums may be 

 partly seen through the widely-open orbits at 9 and 13, 

 indicating the presphenoid and vomer respectively. The 

 most conspicuous parts of the upper jaw are the pre- 

 maxillary, 22, and the maxillary, 21, the latter being 

 edentulous, as in most fishes ; the salmon and trout are 

 examples where No. 21 bears teeth. The shape and 

 slight attachment of those bones relate to the necessity of 

 a movable mouth that can be protruded and retracted, in 

 a class of animals that derive no aid in the prehension of 

 their food from their limbs, which are reduced to fins. 

 The upper bent back part of the premaxillary is called 

 its "nasal branch," and is of unusual length in fishes with 

 protractile snouts, as, e. g.^ the dories {Zeus\ certain wrasses 

 {Coricus\ and especially the sly -bream {Sj)ctriis insidiator 

 of Pallas). In this fish, the nasal branch of the premaxil- 

 lary plays m a groove on the upper surface of the skull, 

 and reaches as far back as the occiput, when the mouth 

 is shut and retracted. The descending branch of the 

 premaxillary is attached by a ligament to the maxillary, 

 and, as this is similarly attached to the mandible, both 

 are protruded, when the long nasal branch of the pre- 



