MALACOPTERYGII 549 



Alhda is represented in the Eocene (London Clay and 

 Bruxellian) ; and the Cretaceous Istieus and Anogmius are believed 

 to be possibly related to Pterothrissus. Four Cretaceous types 

 {Plethodus, Thryptodus, Syntegmodus, and Ancylostylus) are referred 

 with doubt to the Albulidae. 



Fam. 7. Mormyridae. — Margin of the upper jaw formed by 

 the single praemaxillary and the maxillaries, the latter articulated 

 above the former to the ethmoid. Parietal bones separating the 

 supraoccipital from the frontals ; a large hole on each side of the 

 skull, between the squamosal, the epiotic, and the opisthotic, 

 covered by a large, thin, bony plate (the snpratemporal), which 

 may extend over a part of the parietal ; symplectic absent ; 

 suboperculum small and hidden under the operculum, or absent ; 

 interoperculum well developed. Basis cranii simple. No 

 pharyngeal teeth. Opercular bones hidden under the skin ; 

 gill-clefts narrow. Anterior ribs sessile ; epineurals, no epipleurals. 

 Pectorals directed upwards. Yentrals with 6 or 7 rays. Air- 

 bladder communicating with the ear. 



Fig. 329. — Monnyrus cahallus. i nat. size. 



Curious-looking Fishes, very variable in the form of the head 

 and body and in the extent of the fins. Mouth often very small ; 

 teeth in jaws usually few; teeth usually present on the para- 

 sphenoid, working against a similar patch on the glossohyal ; eye 

 covered over by skin, sometimes very indistinct ; scales small, 

 cycloid ; branchiostegal rays 4 to 8. The dorsal and anal fins 

 may be nearly equally developed (Genyomyrus, Gnathonemus) ; or 

 the former (Iformyrus) or the latter (Ryperopisus) are several times 

 the longer. Gymnarchus, Eel-shaped, apodal, and deprived of the 

 caudal fin, lacks the anal fin, the dorsal extending over the whole 

 extent of the body. Some species of Mormyrops show how a form 



