502 PISCES FISHES. 



of the class, but form most remarkable exceptions to the general 

 law in accordance with which the Vertebrata are organized. 



The animals presenting this anomalous configuration are the 

 PlcurontctuLe, or Flat-fishes, as they are generally termed, which 

 when at rest lie quietly upon the ground, where, from the colour 

 of the upper part of their bodies, they are scarcely distinguish- 

 able. To an ordinary observer the Pleuronectidse would seem to 

 have their bodies flattened and spread out horizontally, so that, 

 while resting upon their broad and expanded bellies, their eyes, 

 situated upon the back of the head, are thus disposed for the pur- 

 pose of watching what passes in the water above them ; and this, 

 the vulgarly received opinion, is considerably strengthened by the 

 fact, that what is usually called the belly is white and colourless, 

 while the back is darkly coloured and sometimes even richly varie- 

 gated. The very name used in scientific language to distinguish this 

 extensive family (Pleuronectes*) is calculated to propagate the error ; 

 and few imagine that, in applying the terms back and belly to the 

 upper and under surfaces of a Plaice or a Turbot, they are adopting 

 a phraseology quite inadmissible in an anatomical point of view. 



On examining the skeleton of a Flat-fish, we at once see that 

 what we supposed to be the dorsal and ventral regions are in 

 reality the two sides, which are thus strangely different in colour ; 

 and that the great peculiarity of their structure is the want of 

 symmetry between the lateral halves of the body, arising from the 

 anomalous circumstance that both the eyes are placed upon the 

 same side of the head. Their cranium, indeed, is composed of the 

 same bones as that of an ordinary fish, but the two lateral halves 

 are not equally developed ; and the result is such a distortion of the 

 whole framework of the face, that both the orbits are transferred to 

 the same side of the mesial line of the back. 



The position of the pectoral and ventral fins slightly participates 

 in this want of symmetry, but in other respects the skeleton (Jig. 225) 

 precisely corresponds with that of the generality of osseous fishes. 

 The superior and inferior spinous processes of the vertebrae are 

 amazingly developed, and the interspinous bones (74) of inordi- 

 nate length, so that the vertical diameter of the body is dispro- 

 portionately increased, and the animal is obliged to swim and rest 

 upon one side. The dorsal Jin (75) runs along the whole length 

 of the back ; the anal Jin (a) reaches from the large spines that 

 form the posterior boundary of the abdom'en to the tail, which 

 latter holds the same position as in other tribes ; so that the reader 



a, the side ; vjjx'w, a fin. 



