78 bulletin: museum of comparative zoology. 



warrant for maintaining Dorudon as a separate genus, at least until more 

 material representing D. serratus has been collected. As Basilosaurus 

 cetoides is also, I think, to be considered distinct, on account of its 

 excessively long lumbar vertebrae and very thick epiphyses, the form 

 brachyspondylus minor appears to require a new generic appellation. 

 I would propose the name ZYGORHIZA, and would assign to the genus 

 the characters given in the foregoing column, under Z. brachyspondylus 

 minor. This subspecies is the type of the genus. 



The upper molars of Z. bracliyspondylus minor are smaller than those 

 of D. serratus, the anteorbital region is differently shaped, and the occipi- 

 tal crest is much higher. The last-mentioned character may, however, 

 be due to difference in age or sex. 



The dental formula of Z. bracliyspondylus minor cannot be positively 

 determined from the material at hand. For the lower jaw, however, it 

 appears to be I. 3, C. 1, PM. 4, M. 3. The number of teeth is less im- 

 portant among the zeuglodonts than their form, as the difference in the 

 various genera is not more than one molar tooth above and below. The 

 divergence of the roots of the premolar and molar teeth, which is men- 

 tioned by Dames as a distinguishing character of Dorudon, appears to 

 me of no great value, as the roots of the lower premolars of Z. brachy- 

 spondylus, at least, show a considerable divergence. The size and shape 

 of the accessory cusps of the molars and premolars of D. serratus are not 

 very different from those of Z, bracliyspondylus. In Gibbes's figures of 

 Dorudon, they are too nearly erect and somewhat exaggerated in size. 



