SKELETON OF TELEOSTEI. 91 



the last case the maxillary may be quite abortive. The 

 mobility of the upper jaw is greatest in those fishes in which 

 the premaxillary alone forms its margin. The form of the 

 premaxillary is subject to great variation : the beak oiBelone, 

 XipMas is formed by the prolonged and coalesced premaxil- 

 laries. The maxillary consists sometimes of one piece, some- 

 times of two or three. The principal membrane-bone of the 

 mandible is the dentary (34), to which is added the angular 

 (36) and rarely a smaller one, the splenial or os operculare, 

 which is situated at the inside of the articulary. 



5. Cartilage-hones of the respiratory portion of the visceral 

 sTceleton of the skull. — With few exceptions all the ossifications 

 of the hyoid and branchial arches, as described above (p. 58), 

 belong to this group. 



6. Memhrane-iones of the respiratory portion of the visceral 

 skeleton of the skull. — They are the following : the opercular 

 pieces, viz. operculum (28), suhoperculum (32), and interoper- 

 culum (33). The last of these is the least constant ; it may 

 be entirely absent, and represented by a ligament extending 

 from the mandible to the hyoid. The urohyal (42) which 

 separates the musculi sternohyoidei, and serves for an in- 

 creased surface of their insertion j and finally the hranchio- 

 stegals (43), which vary greatly in number, but are always 

 fixed to the cerato- and epi-hyals. 



7. Dermal hones of the skull. — To this category are referred 

 some bones which are ossifications of, and belong to, the 

 cutis. They are the turhinals (20), the suhorhitals (19), and 

 the supratemporals. They vary much with regard to the 

 degree in which they are developed, and are rarely entirely 

 absent. Nearly always they are wholly or partly transformed 

 into tubes or hoUows, in which the muciferous canals with 

 their numerous nerves are lodged. Those in the temporal 

 and scapulary regions are not always developed ; on the 

 other hand, the series of those ossicles may be continued on 



