X INTRODUCTION. 



pages 305, 811, 314, and 315, although the illustrations in 

 this work are from necessity of small size. Each scale is 

 attached to the skin of the fish by its anterior edge ; and 

 the manner in which the scales overlap each other in differ- 

 ent genera is variable, and gives an appearance of form to 

 each scale which in reality it does not possess. By macera- 

 tion in water, scales exhibit a series of laminse, the smallest 

 in size having been the first produced : they resemble a cone, 

 the apex of which is outwards, the smallest being in the 

 centre ; hence the appearance of numerous concentric lines, 

 all of the same shape, which mark the growth. Scales from 

 the same fish differ in size, depending on the part of the 

 body from Avhich they are taken : those above the lateral line 

 are smaller than those immediately below it, but the scales 

 near the vent are the smallest. 



The row of scales along the side, forming the lateral line, 

 in addition to the structure common to the scales of the 

 other parts of the body, are pierced through near the centre 

 by a tube which allows the escape of the mucous secretion 

 produced by the glands beneath. Each of the various scales 

 represented at pages 5, 33, 339, and 357, exhibit this tube, 

 with the numerous concentric, and some radiating lines, by 

 which the scales of particular species are distinguished. 



The fins are important not only as organs of motion, but 

 as affording by their structure, position, and number, mate- 

 rials for distinguishing orders, families, and genera. The 

 membranes of the fins are thin, and more or less transparent, 

 supported by slender elongated processes of bone, some of 

 which consist of a single piece, which is pointed at the end : 

 such fin-rays are called spinous rays. Others are formed of 

 numerous portions of bone united by articulations, and fre- 

 quently divided at the end into several filaments : these from 

 their pliant nature are called soft or flexible rays, and two 



