112 



portion of the hyomandibula is broader than the rest of the bone. The dorsal margin of 

 the bone is concave and thus forms two articular facets. The anterior facet contacts the 

 hyomandibular fossa which is formed by the sphenotic. pterosphenoid. and the prootic: 

 the posterior one fits into the hyomandibular fossa that is formed by the sphenotic. prootic, 

 and pterotic (Howes 1978. Arratia & Schultze 1991). 



The dorsoposterior margin of the hyomandibular bears an articular facet, articulating with 

 the anterior margin of the opercle. The ventral margin of the hyomandibular contacts the 

 dorsoposterior end of the symplectic, and the anterior margin contacts the surfaces at the 

 posterior margin of the metapterygoid. 



In Phoxinus, the hyomandibular is a plate-like bone. As in other cyprinids. two articula- 

 ting facets are present on its dorsal margin. The hyomandibular is thick at its dorsal and 

 middle part; it is thin at its anteroventral and posteroventral part. Therefore, three parts 

 can be recognized from the bone, i.e., the body (the thick part), the anterior wing (the an- 

 terior thin part), and the posterior wing (the posterior thin part). 



The hyomandibular is broad and relatively short in P. brachyurus, phoxinus. and issykku- 

 lensis (TS 134[0]). It is narrow and relative long in other Phoxinus species (TS134[1]). 

 The bone bears a notch at the ventroanterior margin in P. cumberlandensis and erythro- 

 gaster (TS 1 35 [ 1 ] ): the notch is absent in other species of Phoxinus (TS 135[0]). In P. 

 eos, a notch is present at the ventroposterior margin of the hyomandibular (TS 136[1]); 

 this notch is absent in other Phoxinus species (TS 136[0]). 



A cartilage is present at the ventral end of the hyomandibula, articulating with the sym- 

 plectic. The cartilage is large in P. cumberlandensis, erythrogaster, and eos (TS 1 3 7 [ 1 ] ) : 

 it is much smaller in other species of Phoxinus (TS 137[0]). 



In the outgroups. the short and relatively broad hyomandibula does not bear notches at its 

 ventroanterior and ventroposterior margins. A small cartilage is attatched to the ventral 

 end of the bone. 



Opercular Region 



In cyprinids. the opercular region is located at the most posterior portion of the vis- 

 cerocranium, and forms the lateral cover for the gill arches. It is composed of four der- 

 mal paired plate bones, i.e.. the opercular, subopercular, interopercular, and preopercular 

 bones. 



Opercle (Figs.71A-F). The opercle is the largest bone in the opercular region and slight- 

 ly square-shaped in Phoxinus. The opercle articulates with the hyomandibula and preo- 

 percle anteriorly, the interopercle ventroanteriorly, and the subopercle ventraUy. 

 The shape of the opercle varies among species of Phoxinus. It is elongated and narrow in 

 P. cumberlandensis (TS 138[ 1 ] ): it is short and broad in other Phoxinus species (TS 

 138[0]). The dorsal margin of the bone is almost straight in P. brachyurus, tennesseensis. 

 and cumberlandensis (TS 139[0]): the margin is concave in P. erythrogaster, oreas, eos, 

 neogaeus, issykkulensis. and phoxinus (TS 139[1]). A narrow and sharp anterodorsal pro- 

 cess is present at the anterior end of the dorsal margin in P. cumberlandensis. tennes- 

 seensis, oreas, neogaeus, and phoxinus (TS 1 40[ 1 ] ): the process is broad and blunt in P. 

 erythrogaster, eos, brachyurus. and issykkulensis (TS 140[0]). 



