ACERINA CERNITA. 33 



GrENUS : ACGrma (CUVIER AND VALENCIENNES). 



This is a small group of Perches in which there is but one dorsal fin, and 

 that always has the upper border concave. Its anterior part is formed of 

 spinous rays and its hinder part of soft rays, so that the single dorsal is 

 formed by the blending of the two dorsals of the Perches, owing to the obli- 

 teration of the interval between them. The teeth are similarly villiform, but 

 there are no teeth on the palatine bones. The skull has the system of mucus 

 channels greatly developed in the bones. The operculum and pre-operculum 

 are both spiny. The scales are small. The throat and abdomen have few 

 scales. There are few pyloric appendages to the stomach. 



This genus is limited to the Old World and to its Palsearctic portion, 

 being found distributed over Siberia, and in most, if not all, European 

 countries. 



Acerina cernua (LINN^US). The Ruff, or Pope. 



D. 13 15/12, A. 2/5 6. 



This species (Fig. 12), popularly known in England as the Ruff, or the 

 Pope, has a close general resemblance to the Perch in form. The snout is 

 blunt, but rounded ; the dorsal fin may have twelve or fifteen spinous rays. 

 The fish is about four and a half times as long as high, and about a third 

 higher than thick. The greatest height is over the base of the ventral fin, and 

 is nearly equal to the length of the head. The height of the head is two- 

 thirds of its length ; it is more than half as broad as long. The eye is large, 

 sub-circular, or slightly oval, and placed well to the side, but so as to look a 

 little forward. Its diameter is rather less than one-fourth of the length of 

 the head. The mouth is surrounded by fleshy lips. It descends obliquely, is 

 capable of a slight protraction, and the gape extends at least as far back as 

 the nasal apertures. The tongue is moderately developed. The pre-maxillary 

 bones and lower jaws possess rows of teeth of even size. The vomer 

 carries only a small group of teeth. The nasal apertures are double, and oc- 

 cupied chiefly with a remarkable expansion of the nerve of smell. The 

 anterior aperture is small, round, margined by a valve, and midway between 

 the orbital border and the extremity of the snout. The posterior aperture 

 is large, sub-triangular, and near to the eye. 

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