14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



and pm^ with compressed crown and serrated anterior and posterior 

 margins; m^, ma and mg also with compressed crown, its anterior 

 margin smooth, its posterior margin serrate. Tooth succession 

 occurred in the ordinary way as it assuredly did in Protocetus also. 

 In all characters the skull agrees essentially with that of Protocetus. 

 The basal part of the occipital appears, however, to be more adapted 

 to the larynx and gullet. A few peculiarities of Prozeuglodon which 

 are not clearly demonstrated in Protocetus are : the presence of an 

 elongated, compressed incisive foramen on each side ; the presence 

 of a distinct lacrimal bone ; and the presence to the outside of the 

 pterygoid and in front of the tympanic of a considerable pit bounded 

 by high ridges springing from the surrounding bones, evidently the 

 impression of an air-sac. The under jaw already has nearly the same 

 peculiar form as in many highly developed cetaceans with long 

 symphysis menti ; it has, however, a relatively large coronoid process. 

 But the mandibular condyle is placed low and is turned backward, 

 and the strange gaping hinder entrance to the mandibular canal is 

 present. Of the rest of the skeleton rather more is known than of 

 Protocetus, among other parts most of the vertebral column and the 

 fore limb down to the hand. There is a great similarity to Protocetus. 

 A difference from this genus is that no sacral vertebra is found with 

 the transverse process plainly acted upon by the ilium. The skeleton 

 of Prozeuglodon throws light on certain conditions that are not under- 

 stood in Protocetus. The sternum is of considerable size, with several 

 joints. The shoulder blade is essentially as in the higher whales. 

 The humerus has retained relatively much of the original form : 

 distinctly separated greater and lesser tubercles, a distinct deltoid 

 crest, and a well-developed hinge-shaped lower articular surface. 

 Radius and ulna have correspondingly well-developed articular sur- 

 faces for the humerus, are relatively only a little compressed, and 

 have distinct articular surfaces for the carpal bones ; the ulna has a 

 rather large olecranon. 



Zeuglodon \_Basilosaurus'\ (Z. cetoides, Z. isis), known rather 

 completely as to the skeleton, occurs in Eocene strata of both the 

 Old and New Worlds. In most respects it resembles Prozeuglodon. 

 But it has acquired a highly remarkable peculiarity in the vertebral 

 column. While the centra in Prozeuglodon are not in any direction 

 strikingly altered in form, in Zeuglodon the centra of most of the 

 hinder thoracic vertebrae, of the lumbar vertebrae, sacral vertebrae and 

 all but the outermost of the caudal vertebrae, have become remarkably 

 large and especially greatly elongated, while the vertebral arches 

 have remained short, standing about midway on the centra, the arches. 



