HEEEINGS. 655 



Hetekotis. — Cleft of the mouth rather small, with the jaws 

 subequal ; barbels none. A single series of small teeth in the 

 jaws ; pterygoids and hyoid with a patch of small conical teeth ; 

 none on the vomer or palatines. 



This fish {M. niloticus), which is not uncommon in the 

 Upper Nile and the West African rivers, exhibits several 

 anatomical peculiarities. The fourth branchial arch supports 

 a spiral accessory organ, the function of which is still 

 unexplained. The air-bladder is cellular, and the stomach 

 consists of a membranous and a muscular portion. 



Twenty-Second Family — Clupeid^. 



Body covered with scales; head naked; harbels noTie. 

 Ahdo7nen frequently compressed into a serrated" edge. Margin 

 of the upper jaw formed hy the intermaxillaries mesially, and 

 hy the maxillaries laterally ; maxillaries composed of at least 

 three movable pieces. Opercular apparatus complete. Adipose 

 fin none. Dorsal not elongate ; anal sometimes very long. 

 Stomach with a blind sac ; pyloric appendages numerous. Gill- 

 apparatus much developed, the gill-openings being generally very 

 wide. FseudolranchicB generally present. Air-bladder more 

 or less simple. 



The family of " Herrings " is probably unsurpassed by any 

 other in the number of individuals, although others comprise 

 a much greater variety of species. The Herrings are princi- 

 pally coast-fishes, or, at least, do not go far from the 

 shore ; none belong to the deep-sea fauna ; scarcely any 

 have pelagic habits, but many enter or live in fresh waters 

 communicating with the sea. They are spread over all the 

 temperate and tropical zones. Fossil remains of Herrings are 

 numerous, but the pertinence of some of the genera to this 

 family is open to serious doubts, as the remains are too 

 fragmentary to allow of determining whether they belong to 

 Salmonoids or Clupeoids. Therefore, Agassiz comprised both 



