VERTEBRATA: FISHES. 



465 



membrane," which is attached to the os hyoides. The membrane is 

 supported and spread out by a number of slender curved spines, which 

 are attached to the lateral branches of the hyoid bone, act very much 

 as the ribs of an umbrella, and are known as the ' ' branchiostegal rays" 

 (fig. 249, d}. 

 The hyoid arch of fishes is attached to the temporal bones of the skull 



Fig. 249. Skull of Cod (Morrkua viilgarls) Cuvier. a Urohyal; b Basihyal; c 

 Ceratohyal; d Branchiostegal rays ; / Prae-operculum ; o Operculum proper; s Sub- 

 operculum ; i Inter-operculum ; w Mandible ; n Inter-maxillary bone. 



by means of two slender styliform bones, which correspond to the styloid 

 processes of man, and are called the " stylohyal " bones (fig. 250, /). The 

 rest of the hyoid arch is composed of a central portion and two lateral 

 branches. Each branch is composed of the following parts : I. A tri- 

 angular bone attached above to the stylohyal, and termed the " epihyal 

 bone" (fig. 250, e) ; 2. A much longer bone, known as the "ceratohyal" 

 (d). The central portion of the hyoid arch is made up of two small poly- 

 hedral bones the "basihyals" (b). From the basihyal there extends 

 forwards in many fishes a slender bone, which supports the tongue, and is 

 termed the " glossohyal " or "lingual" (a). There is also another com- 

 pressed bone which extends backwards from the basihyals, and which is 

 known as the "urohyal bone" (c). This last-mentioned bone is of im- 

 portance, as it often extends backwards to the point of union of the cora- 

 coid bones, and thus forms the isthmus which separates the two branchial 

 apertures. 



From the outer margins of the epihyal and ceratohyal bones on each side 

 arise the slender curved "branchiostegal rays," which have been previously 

 mentioned. There are usually seven of these on each side. Above the 

 urohyal, and attached in front to the body of the os hyoides, is a chain of 



2 G 



