PREFACE. XV 



suring a large number of individuals. In the Class of Birds, 

 they are generally those of the male.) unless stated to the 

 contrary. It may be observed that in this Class the entire 

 length is measured from the tip of the bill to the ex- 

 tremity of the tail; the length of the bill is estimated in 

 two directions, firstly from the frontal feathers, following 

 the curvature of the ridge, secondly from the gape or 

 angle of the mouth ; the length of the tail is measured 

 from the extremity of the longest quill to its insertion in 

 the coccyx. 



Fish being very variable in size, and having no very 

 well marked limit of growth, it was thought that the 

 absolute dimensions of any species, would, if given, prove 

 of little value. Hence in this Class, the author has gene- 

 rally restricted himself to noting the average length to 

 which the species attains; the relative proportions, which 

 are often of great importance, being introduced into the 

 body of the description. With reference to these propor- 

 tions, it may be stated, that the entire length, unless men- 

 tioned to the contrary, is always measured to the extremity 

 of the caudal fin. The length of the head is measured from 

 the end of the snout to the posterior margin of the gill-cover. 

 The depth (termed the breadth in the case of the Pleuro- 

 nectidce) is reckoned vertically from the most elevated point 

 in the line of the back, termed the dorsal line, to the cor- 

 responding point in the line of the abdomen, or ventral line, 

 the dorsal and anal fins, unless mentioned to the contrary, 

 not being included. 



It may be added that in computing the fin-ray formula, 

 which is similar to that adopted by MM. Cuvier and Va- 

 lenciennes, all the rays are included which it was possible 

 to distinguish, on the ground that if the short rays be 

 omitted, which often pass insensibly into the longer ones, 



