HISTORY OF WHALES 



39 



premolar, possess a well defined cingulum 

 and retain distinct vestiges of the inner 

 portion of the crown. The premolars 

 have a crown with one large cusp and a 

 rudimentary basal posterior cusp. The 

 fourth lower premolar has the longest 

 antero-posterior diameter. Each of the 

 three molars has a large anterior cusp 



1 and a smaller posterior cusp. In contrast 

 to zeuglodonts of later stages the second 



l incisor is relatively large. 



In passing from the lower Middle 

 Eocene to the uppermost Eocene, zeuglo- 

 donts are encountered which reveal some 

 of the progressive stages in the remodel- 

 ling of the cheek teeth as well as the 

 reshaping of the occipital region of the 

 skull and other skeletal changes. 



Several faunas of these zeuglodonts have 

 been discovered in the Middle and Upper 

 Eocene deposits of the Fayum in Egypt. 

 These rocks are older than our own Gulf 

 Coastal Plain deposits and have yielded 

 several rather distinct types of zeuglo- 

 donts. The oldest geologically is a some- 

 what generalized form, Protocetus atavus, 

 described by Fraas in 1304, from the basal 

 member of the Lower Mokattam stage 

 near Cairo. The skull of this small 

 species is less than two feet in length and 

 bears little resemblance to living cetaceans . 

 The normal eutherian dentition is present. 

 In common with the oldest land mammals, 

 there is a marked difference between the 

 anterior grasping teeth and the posterior 

 cutting teeth. In contrast to zeuglo- 

 donts of later stages of the Eocene, the 

 teeth of Protocetus do not bear step-like 

 cusps on their anterior and posterior 

 cutting edges, and distinct vestiges of the 

 inner portion of the crown are retained. 

 The two posterior upper premolars and 

 the three molars have three roots and a 

 remnant of the postero-internal cusp. 

 Both Fraas and Andrews have pointed out 

 that Protocetus is a highly important link 



in the history of the Archaeoceti, for here 

 we have a skull that is typically zeuglo- 

 dont in general form combined with a 

 dentition that is essentially that of a 

 creodont. In some characters, such as 

 the peg-like odontoid and the relatively 

 long centrum, the axis of the lower Middle 

 Eocene Protocetus approaches the type of 

 cervical found in the carnivores. The 

 late Dr. C. W. Andrews of the British 

 Museum of Natural History contended 

 that the zeuglodonts probably originated 

 on the northern shores of Africa in the 

 early part of the Tertiary period and that 

 Protocetus was an annectant form between 

 the later zeuglodonts and the earlier 

 creodonts. 



In the upper Middle Eocene a slightly 

 more advanced type of zeuglodont, Pro- 

 Xeuglodon atrox (Andrews, 1906) makes its 

 appearance. It should be noted here that 

 Professor Stromer (1308) has suggested 

 that this may be a young individual of 

 Zeuglodon isis. The skull of this species 

 measures about 2.6 inches in length, and 

 the occipital shield has the appearance of 

 being less constricted above the condyles. 

 The number of molars is uncertain, but 

 otherwise it has the normal eutherian 

 dentition. The cutting edges of the 

 crowns of the molars and the three poste- 

 rior premolars are serrated. Pm. 3 , Pm. 4 , 

 and M. 1 have an enamel covered buttress, 

 evidently the remnant of the postero- 

 internal cusp, and in the premolars at 

 least this buttress is supported by a dis- 

 tinct third root. Pro^euglodon has an 

 axis with a short, blunt, rounded odon- 

 toid, while that of "Zeuglodon ' is flattened 

 above. Zeuglodonts of the Upper Eocene 

 have an axis with a high massive neural 

 spine, but the centrum and transverse 

 processes are shortened. The evidence 

 appears to be fairly clear that a shortening 

 of the neck was brought about by a re- 

 duction of the centra of the cervical 



