THE TELEOSTEI. 157 



no brancliial filaments, but commonly support teeth, and 

 are called hypoplianjngeal bones. In certain osseous fishes, 

 thence called FhanjngognatJii, they ankylose together into 

 one bone. The anterior foiu- pair are composed of several 

 joints, and the uppermost articulations of more or fewer of 

 them usually expand, bear teeth, and form the epipharyngeal 

 bones. Sundry important membrane bones are connected 

 with the mandibidar and hyoidean arches. The preoper- 

 culum {P.Op.), operculum [Op.), and branchiostegal rays [Br.]. 

 jih-eady met with among the Ganoidei, are the most constant 

 of these. Beneath the operculum, lies a suboperculum(S.Op.), 

 and below this an interoperculum (I.Op.), which is con- 

 nected by ligament with the angular piece of the lower jaw, 

 and is also imited to the outer face of the hyoidean arch. It 

 may be altogether ligamentous, as in the Siluroids. 



The branchiostegal rays are attached partly to the inner, 

 and partly to the outer, surface of the hyoidean arch. They 

 support a membrane, the branchiostegal membrane, which 

 serves as a sort of inner gill-cover. 



Most Teleostei possess two pair of limbs, the pectoral and 

 the ventral fins. But the latter are often absent, and the 

 former are occasionally wanting. When the pectoral fins 

 are absent, the pectoral arch usually remains, though it 

 may be reduced to little more than a filament, as in Murcen- 

 ophis. The ventral fins are frequently situated in their nor- 

 mal position beneath the posterior part of the trunk ; but in 

 considerable groups of these fishes they are immediately 

 behind the pectoral fins (thoracic), or even in front of them 

 (jugular). In the asymmetrical Pleiironectidce one pectoral 

 fin may be larger than the other, or may alone remain, as 

 in Monochir%i.s. 



The pectoral arch always consists of a primarily cartila- 

 ginous coraco-scapular portion — which usually ossifies in two 

 pieces, a coracoid below, and a scapula above — and of sundry 

 membrane bones. The chief of these membrane bones is 

 the clavicula {CI), which meets its fellow in the middle hne. 

 and is usually joined to it by ligament, but sometimes, as 



