CEAJS'IAL OSTEOLOGY OP THE PHEACTOI-^MID^. 281 



the reduction of the dentitiou, and in the forward dislocation 

 of the quadrate, Pliractolaemus resembles Chanos. In Chanos the 

 relations of the quadrate to the mandibular ramus are normal, 

 but in consequence of the modification of the mouth-parts the 

 quadrate is so far thrust forward as to have parted from the 

 metapterygoid, a most unusual condition. The pr&maxillse of 

 Chanos are thin scales, like those of PhractolcBmvs ; these alone 

 bound the gape above, but if Chanos were capable of opening its 

 mouth as widely as Phractolcemus, the hinder parts of themaxillge 

 w^ould bound the sides of the gape. 



The main objections to the association of Pliractol(emu8 with 

 Chanos lie in the possession by the latter of a strongly developed 

 horizontal limb of the preopercular, a fully roofed posterior 

 temporal fossa of large size, backwardly directed spines projecting 

 from the squamosal, supraoccipital, and exoccipital bones, an 

 ossified first basibranchial, and two hypohyals on each side, and 

 in the small size of its vomer. These objections are not insuper- 

 able, however, and the resemblance between the skulls of Phrac- 

 tolcemus and Chanos, though possibly due to " Convergence," is 

 sufficiently suggestive to warrant the undertaking of a com- 

 parative study of the other parts of the body of these fishes 

 by any one having the necessary time and material at his 

 disposal. 



EXPLAXATIOX OF THE PLATES. 

 Plate 30. 



Fig. 1. OsteoglossumLeichardti. Dorsal Tiew of crani am. 



2. „ „ Side view of cranium. 



3. „ „ Back riew of cranium. 



4. „ „ Eight side of skull. 



5. „ ,,. Hyopalatine arch of left side, with opercular 



bones and mandible, mesial aspect. 



Plate 31. 



Fig. 6. Heterotis nihtkus. Dorsal view of cranium. 



7. „ ,, Side view of cranium. 



8. ,, „ Back view of cranium. 



9. „ „ Eight side of skull. 



10. „ ,; Hyopalatine arch of left side, with opercular 



bones and mandible, mesial aspect. 



11. „ „ Occipital half- vertebra, posterior view. 



