102 A BOOK OF WHALES 



that this has really to do with the mode in which the 

 Rorquals and Right whales feed. The capacity for 

 taking in enormous gulps of water containing the 

 minute animals upon which the majority of these 

 whales feed would be advantaged by a distensibility 

 of the mouth, and a consequent increase in size of 

 the mouth cavity. Of more importance in connection 

 with the anatomy of the lower jaw is the discovery 

 by Professor Albrecht of a separate supra-angular 

 bone. It is a distinguishing feature of the mammals, 

 as contrasted with the reptiles lying beneath them 

 in the series, that the lower jaw is almost entirely 

 formed of a dentary bone alone (a small chin bone 

 sometimes occurring also). Now in reptiles a large 

 number of separate elements enter into its formation, 

 so that the occasional occurrence in Balcenoptera 

 sibbaldii of the supra-angular is so far an archaic 

 feature. So too, possibly, is the marked separation 

 of the sternum into two hemisterna. This is particu- 

 larly apparent in the Cachalot and in the Ziphioids. 

 Now the sternum is developed from the ends of the 

 ribs on both sides, and in the embryo it is always 

 double ; later the fusion of the two halves takes place, 

 and the apparently median- sternum arises. In lower 

 vertebrates the double condition often survives. 



That there is often a seventh cervical rib in whales 

 is a remnant of a former state of affairs ; for in 

 reptiles there are a series of ribs depending from the 

 neck vertebrae. But after all such an additional rib 

 has been often met with in other mammals. Professor 

 Albrecht points out that the Cetacea resemble the 



