CEAjSTAL osteology of the OSTEOGLOSSIJDiE. 259 



of O. Leichardti and 0. hicirrhosum. The branchiostegal rays are 

 14 or 15 ill number on each side : according to Miiller and 

 Schlegel (Temminck's Yerh. Nat. Nederl. overz. bez., Zool.' 

 Leideu, 1839-44, Pisces, p. 7) the number is 15 ; Hyrtl 

 (Denkschr. Akad. Wiss. Wien, viii. 1854, p. 76) says 17 ; 

 Giinther (Brit. Mus. Cat, Fisb. vii. 1868, p. 378) says 15; 

 Boulenger (Ann. Mag. Nat. Hist. (7) viii. 1901, p. 515) says 

 15-17. 



Heteeotis niloticus. 



Hyrtl (Denkscbr. Akad. Wiss. Wien, viii. 1854, pp. 73-76, three 

 plates) has published a side view of the skull oi Seterotis, and a 

 moderately good description, not sufficiently detailed, bowever, 

 to be of much real service. An account of the byobranchial 

 skeleton is also given, and bis figures of the epibranchial organ, 

 the shape of vphich he compares with that of the shell of Planorbis, 

 show well the peculiar features of this remarkable structure. 



Views of the skull, gills, and epibranchial organ are also given 

 by Heraprich and Ehrenberg (Symbolse Physicse, zootomical 

 plates 8 and 9, 1899). 



The skull described below is that of a skeleton in the British 

 Museum. It is marked " Kartoum," but bears no register 

 number. 



Cranium (PI. 31. figs. 6, 7, and 8.) — The nasal bones are large 

 and are incorporated intjo the cranium ; they meet one another 

 in a median suture, and are suturally united with the frontal 

 bones. The mesethmoid is small, and appears on the dorsum of 

 tbe cranium between the anterior ends of the nasal bones. The 

 frontals are large and broad ; the parietals are comparately large 

 and meet in a median suture. The nasal, frontal, squamosal, and 

 parietal bones are sculptured, but the hindermost parts of each 

 parietal and squamosal lie at a deeper level and are not sculp- 

 tured. Shallow unsciilptured depressions, each with a perfora- 

 tion by which the sensory canal comes to the surface of the head, 

 occur on the nasal, frontal, parietal, and squamosal bones in the 

 positions shown in fig. 6. The supraoccipital is small and has a 

 feeble crest, and the posterior part of the cranium lying beneath 

 the supraoccipital and above the foramen magnum is cartilaginous 

 (fig. 8). _ _ _ 



The occipital half- vertebra comes away readily from the ex- 

 occipitals and basioecipital, and leaves their posterior ends rough. 

 It has a complete neural arch, but no neural spine ; it bears a 



18* 



