140 PROCEEDINGS OF THE NATIONAL MUSEUM vol. ue 



The opisthotic is a small tabular boue applied to the joiut between 

 the pterotic and exoccipital but lying mostly on the pterotic. 



Dorsally the pterotic is overlaid by the parietal and posteriori}'- 

 contacts the epiotic. Anteriorly and dorsally the pterotic does not 

 reach the frontal but is completely bounded by the sphenotic, while 

 medially it contacts the prootic. Except for lacking a dermopterotic 

 laterosensory canal, for not contacting the frontal, and except for its 

 general shape, the pterotic is much like that of the Characinae. The 

 pterotic of the Pyrrhulinina is like that of the Nannostomina in 

 lacking a laterosensory canal and in not being jointed with the frontal. 

 The Lebiasinini and the Erythrininae have a laterosensory canal in 

 the pterotic and the frontal contacts the sphenotic. 



The sphenotics are much like those of the Characinae; their various 

 relationships to other bones have been described under the frontals 

 and parietals. Their relationships to the semicircular canals are 

 the same as in the Characinae. 



In their essential relationships to other bones and the soft anatomy, 

 the prootics are not greatly different from those of the Characinae. 

 Detailed comparison of the various foramina of this bone with the 

 foramina of the prootics of the Pyrrhulinini, Lebiasinini, and Ery- 

 thrininae undoubtedly would prove productive in determining rela- 

 tionships; however, this work must be postponed pending detailed 

 studies of the nerves, veins, and arteries passing through this com- 

 plicated bone. 



The subtemporal fossa is present as a shallow groove along the 

 joint between the prootic and pterotic and is present in all members 

 of the Erythrininae and Lebiasininae. 



The pterosphenoid of Poecilobrycon harrisoni and other species of 

 the Nannostomina is similar to that in the Characinae, being a large 

 plate in the upper posterior wall of the orbit. The foramen for the 

 trochlear nerve is located at the junction of the articulation between 

 the frontal, pterosphenoid, and orbitosphenoid. 



The orbitosphenoid is Y-shaped in cross section; the upper arms 

 of the Y are jointed to the frontals. A foramen for the first cranial 

 nerve is enclosed partially by the orbitosphenoid. Other species in 

 the Nannostomina examined also have an orbitosphenoid that is 

 Y-shaped in cross section. The same is true of the Pyrrhulinina. 

 In the Lebiasinini the cross sectional shape of the orbitosphenoid is 

 more like a V than a Y. In Hoplias the foramen for the first cranial 

 nerve is included completely in the orbitosphenoid of adults but 

 included only partially in that of the young. Erythrinus has a rather 

 peculiar orbitosphenoid, consisting of a pair of bilateral flat plates 

 attached ventrally by cartilage. In specimens larger than those 

 examined these plates possibly may fuse ventrally, producing a more 



