TRANSACTIONS OF THE SECTIONS. 619 



the name of caecum. The swimming-bladder of all of them is 

 very large, and opens into the oesophagus near the bottom of 

 the gullet. Though unable here to enter into the subject 

 very fully, the author states his persuasion that this organ 

 ought to be regarded as the lungs of fishes ; that the cir- 

 culation of the blood in these animals has been inaccurately 

 interpreted when it is supposed that in their heart there may 

 be traced a pulmonary course ; also, when their branchiae have 

 been identified with the lungs of other animals ; and, finally, 

 when their great dorsal artery has been considered as analo- 

 gous to the aorta of the mammalia. 



Most of the varieties of salmon reside in fresh waters ; in 

 summer they pay a visit to the sea, and do not mount up again 

 to the rivers, except for the purpose of there depositing their 

 spawn. It is sufiiciently remarkable that most of our species 

 deposit their ova in November and December, and that the 

 young fry of course come into existence in the coldest season of 

 the year. From this circumstance we may suppose that it is 

 owing to this habit of enduring intense cold in the first days of 

 their existence, that they can subsequently support all that va- 

 riety of temperature to which they are soon to be exposed. 



In proportion as the genus Salmo is now circumscribed within 

 its natural limits, so much the more is it difiicult to characterize 

 the various species ; and M. Agassiz afiirms without hesita- 

 tion, that since no one has devoted himself to their history, so 

 no one has yet succeeded in determining, with any degree of 

 precision, their distinctive characters. The greatest obstacle to 

 the solution of this problem arises from our ignorance of the 

 accuracy of the characters hitherto employed to distinguish the 

 several species the one from the other. 



Naturalists have especially attached themselves to the form 

 of the head and to the arrangement of the colours ; but these 

 two particulars are much too variable to supply precise charac- 

 ters. As to the variation in the colour, we may say it is infinite. 

 There are, however, two circumstances which especially modify 

 the tints of the salmon tribes, namely, their age and the season of 

 the year. Tlie younger fish are in general much more spotted 

 than the older ones, whose tints become more and more uni- 

 form. The Salmo Hucho, for example, with violet spots more 

 or less distinct, has, when young, large black transverse bands 

 upon the back down to the middle of its sides. In the second 

 and third years of its existence these bands break up into black 

 spots, less deep in colour, and they disappear more and more, 

 till in its latter years the fish acquires a colour which is almost 

 uniform. The Salmo lacustris of Linnaeus, when young, has 



