THE AIORMTKID^, WOTOPTERID^, AND HTODONTIDJS. 201 



first pharyngobranchial ; the second and third are quite short, 

 the second being triangular and the third square in general 

 outline. The pharjngobranchials of the right and left sides are 

 rather widely separated, and there intervenes a considerable 

 expanse of pharyngeal mucous membrane unsupported by 

 skeletal parts. 



Distinctive Features of the Mormyroid Skull. 



The following features, present in the seven genera Mormyrops, 

 Petrocephalus, Marcusenius, Gnatliostomus, Hyperopisus, Mor- 

 myrus, and Gymnarchus, will doubtless prove to be common 

 throughout the family Mormyridae. 



The parietal bones meet in a median suture, and are not 

 separated by the supraoccipital ; an orbitosphenoid is present, 

 but no opisthotic ; there is a large lateral cranial foramen, 

 bounded by the squamosal, epiotic, and exoccipital, and loosely 

 overlaid by a thin, scale-like supratemporal * ; there is no 

 posterior opening to the eye-muscle canal. There are no teeth 

 on the maxillary, vomerine, palatine, and pterygoid bones. 

 There is a very extensive attachment of the whole of the upper 

 edge of the hyopalatine arch with the cranium; there is no 

 separate symplectic, and no separate entopterygoid. The pala- 

 tine is fused with the side of the vomer ; the right and left 

 premaxillse are fused together, although the suture may remain 

 visible, as in Gymnarchus ; there is no surmaxilla. The sub- 

 opercular terminates in a point posteriorly, and, except in 

 Gymnarchus, is entirely concealed by the opercular. The hypo- 

 branchials are greatly reduced, and a pair of large tendon-bones 

 project downward and outward (horizontally outward and back- 

 ward in Gymnarchus) from the hypobranchial region. There is 

 no separate glossohyal ; it is either wanting {Petrocephalus), 

 or is fused with the first basibranchial. The urohyal is without 

 the usual paired ligament in front ; it is rigidly fixed, or even 

 fused {Gymnarchus), beneath the anterior part of the copular 

 skeleton. 



* The extensive overlapping of the parietal by the supratemporal, given by 

 Boulenger (Ann. Mag. Nat. Hist. [7] xiii. 1904, p. 164) as a family character, 

 is fallacious. In the Mormyrida the supratemporal covers very little of the 

 parietal bone ; in some species, e. g. Mormyrops deliciosus aiid Gymnarchus 

 niloticus, it does not even reach the parietal. 



