ON MODIFICATIONS IN THE STKUCTUKE OF DIVING ANIMALS. 63 



to the condition of total absence which we find in carnivorous 

 cetacea ; and, but that some of the latter class possessed olfactory 

 bulbs, a similar relation might be said to prevail between these 

 organs also in the two orders. In both classes alike, the weight of 

 the brain was high as compared with that of the body ; in a young 

 Phoca vitulina I have found it to be as 1 : 46, in a young Phocaena 

 comyuunis as 1 : 60. 



The bark of the seal spoke plainly enough to its want of any 

 such arrangement of the larynx as the whales possess ; but a recent 

 inspection of a large seal (Pelagius monachus) had shown it to 

 possess an exceedingly strong sphincter muscle guarding the 

 entrance to the respiratory passages, and it might be conjectured 

 that the membrano-muscular pouch in connexion with the nasal 

 passages in the Stemmatopus cristatus was a foreshadowing of 

 the sac so often described in connexion with the cetacean blow- 

 hole. 



Several foetal structures were permanently retained in the cetacea. 

 The thymus, as shown by Mr. Turner ('Edin. Roy. Soc. Trans.,' 

 xxii. p. 2), was one of these ; certain other remnants of the general 

 formative mass of blastema which surrounds the aorta in the 

 foetus, noticed by me in the 'Natural History Review' for Oct. 

 1 86 1 \ furnished a second example, and to these I would now add 

 a third, in the largest remnant of a Wolffian body, or organ of 

 Giraldes, which I have met with in the class mammalia. I may 

 add that, in the two classes of birds which I have had to contrast, 

 scarcely any such approximations could be traced between the two 

 sets of structures to be compared. 



The modifications in the tibiae of the birds commonly known as 

 'divers' (Colymbinae) , and the large intra-hepatic venous sinuses 

 which they, in common with the mammals just spoken of, possessed, 

 were beautiful adaptations to the special habits of these animals ; 

 but nothing at all reminding us of these structures would be found 

 in such a bird as the water-ouzel (Cinclus aquaticus). Indeed, the 

 soft parts of this bird presented very few points of difference from 

 those of a redwing (Turdus iliacus) dissected at the same time with 

 it, except in the much greater development of the second pectoral 

 muscle. The large size of this muscle was permanently recorded on 

 the keel of the ouzel's sternum ; and this point might perhaps have 



1 Article IV. in this volume. 



