102. 



THE QUARTERLY REVIEW OF BIOLOGY 



taking surface-swimming crustaceans or 

 for impounding small fish, which are 

 scooped up by the animal when plunging 

 forward with mouth open. The capacity 

 of the mouth was increased in two ways. 

 In the balaenids it has been accomplished 

 by arching the upper jaws and in the 

 others by a bowing outward of the lower 

 jaws in conjunction with a broadening of 

 the rostrum. Surprising as it may seem 

 four barrels of shrimps were taken from 

 the partially filled stomach of a huge 

 blue whale (Andrews, 1916), whose throat 

 measured about 9 inches in diameter. 

 It is worth while then to explain just 

 what has been accomplished by the 

 remodeling of the generalized mysticete 

 skull. This brings us around again to a 

 comparison of living and extinct types. 

 In general it may be stated that the 

 forward movement and expansion of the 

 supraoccipital shield was followed or 

 accompanied by a broadening of the base 

 of the skull. This forward over thrust 

 of the posterior occipital elements also 

 changed the relative positions of the 

 zygomatic processes, for they become 

 more robust and their articular surfaces 

 come to lie near the level of the occipital 

 condyles. The mouth cavity increased in 

 size, and this in turn permitted a lengthen- 

 ing and expansion of the blades of baleen 

 that hang from the roof of the mouth. 

 The mandibles acquired a disproportion- 

 ately large size in comparison with the 

 braincase, becoming bowed outward and 

 loosely connected anteriorly. The sym- 

 physis disappeared, and the ligaments 

 that bind the articular head of the man- 

 dible to the glenoid articular surface 

 developed the strength necessary to bear 

 the great weight of the outbowing lower 

 jaws . The opening leading to the nostrils 

 or "blow holes" was drawn backward in 

 proximity with the anterior wall of the 

 braincase. The posterior position of the 



blow-holes permitted exhalation and in- 

 halation without elevation of the muzzle 

 above the water level, which was rendered 

 difficult, if not impossible, by reason of 

 the inflexibility of the neck. The increase 

 in the size and proportions of the skull, 

 the outbowing of the lower jaws, and the 

 development of long blades of baleen 

 attached to the roof of the mouth, required 

 a firm socket to support the enormous 

 increase in the weight of the head sus- 

 pended from it. This result could be. 

 secured by the development of massive 

 cervicals, or by the shortening of the 

 neck and by the ultimate fusion of the 

 seven cervicals into a compact mass as in 

 Balaena mystketus and Eubalaena facialis. 

 The ankylosis of the cervical vertebrae and 

 their fusion into a compact immovable 

 mass is believed to be the culmination of 

 this evolutionary trend. In the case of 

 any pelagic mammal having feeding habits 

 similar to those of the whalebone whales, 

 flexibility of the neck would be a mechani- 

 cal weakness; hence the short neck appears 

 to fulfill the necessary requirements for 

 cetaceans with relatively large skulls. 

 The basicranial region was largely pro- 

 tected from telescoping by the relations of 

 the maxillary and frontal, and conse- 

 quently the nasal cavity retains a cribri- 

 form plate with passages for the olfactory 

 nerves as well as remnants of the turbi- 

 nated bones, including the ethmoturbinals. 

 It must be understood that this summary 

 attempts to account merely for the general 

 features of the evolutionary history of the 

 mysticetes, and that a late stage in this 

 process may appear at an early geological 

 time or an early stage may persist to the 

 present day. The skull of the living 

 gray whale Qkhachianectes glaucui) is 

 constructed along the lines of an early 

 stage and has been called a living fossil. 

 Conversely the skull of the Lower Miocene 

 Morenocetus -parvus has already acquired 



