138 PROCEEDINGS OF THE NATIONAL MUSEUM vol. ue 



The toothless prevomer is concave ventrally and concave dorso- 

 laterally, being an inverted Y-shaped bone in cross section. In small 

 specimens of Poecilobrycon harrisoni and in the large adults of some 

 species (Nannostomus beckfordi and N. trifasciatus) , the prevomer is 

 formed of three thin concave plates of bone adhering to, or about, 

 a central cartilaginous core. The two fused dorsal plates contact the 

 ethmoid by a cartilaginous (synchondral) joint and posteriorly by 

 another synchondral joint to the parasphenoid and lateral ethmoid. 

 There is no rhinosphenoid. The posterior shaft of the prevomer, 

 which ordinarily contacts the parasphenoid in the characinae, is ex- 

 tremely short and does not reach the parasphenoid. There are a 

 number of foramina on the ventral surface of the prevomer. Many of 

 these are probably for branches of the ramus buccalis facialis nerve. 



Members of the Erythrininae, Lebiasinini, and Pyrrhulinina have 

 a prevomer essentially like that of the Nannostomina; however, the 

 shaft of the vomer is better developed in groups other than the Nan- 

 nostomina and extends well back onto the parasphenoid. Except for 

 the reduced prevomerine shaft in the Nannostomina, the prevomer of 

 the Lebiasininae is essentially like that of the Characinae. 



Each lateral ethmoid projects downward from under its respective 

 frontal and contacts its counterpart at the median vertical plane by 

 a nonmovable synchondral (cartilaginous) joint. The foramen for 

 the olfactory nerve is near the median edge of the lateral ethmoid. 

 An upper medial blade of the lateral ethmoid extends anteriorly and 

 medially to contact a vertical median wall of cartilage behind the 

 prevomer and there forms a nonmovable synchondral joint. Antero- 

 ventrally the lateral ethmoid has a process extending forward to contact 

 a sheet of cartilage that extends anteriorly to the lower portion of 

 the prevomer. Ventrally the prevomer is in contact through cartilage 

 with the parasphenoid and posteromedially with the orbitosphenoid. 

 The lateral ethmoid of all members of the Nannostomina and other 

 members of the Lebiasininae is essentially the same. In the Pyr- 

 rhulinina and the Lebiasinini the distance between the prevomer and 

 the lower medial portion of the lateral ethmoid is much shorter. That 

 of young specimens of Hoplias is similar to that in the Nannostomina, 

 but in older specimens the two bones meet each other. The structural 

 configuration of the lateral ethmoid in the Erythrininae and Chara- 

 cinae is not markedly different in the two groups, and those differences 

 in form that do occur can be correlated with differences in the shape 

 of mouth and snout and their functions. 



The frontals of Poecilobrycon harrisoni are large, smooth, gently 

 curved bones. They are similar to the frontals of the Characinae in 

 their basic structm-al relationship to other bones of the cranium; 

 however, the frontal-parietal fontanel is never present in adults and 



