Mr. 0\VEN on the Zeuglodon Cetoides. yj 



that the superior arch was continuous with the body of the vertebrne, as is always 

 the case in the mature Mammaha. 



Another argument for the mammiferous and cetaceous character of the Zeu- 

 glodon may be drawn from the great capacity of the canal for the spinal chord, 

 which, in the Cetacea, is surrounded by an unusually thick plexiform stratum of 

 both arteries and veins. The cetaceous character is further manifested in the short 

 antero-posterior extent of the neurapophyses, as compared with that of the body 

 of the vertebra, in their regular concave posterior margin, and the development 

 of the articular apophyses only from their anterior part. 



In one of the caudal vertebrse of the Hyperoodon, which measured 8^ inches in 

 length and 4f inches in height, the base of the superior arch was four inches in 

 longitudinal extent, and occupied the same relative position as in the Zeuglodon. 

 In the short-tailed Plesiosaurs the bodies of the vertebrse are always shorter in 

 proportion to their breadth, and the superior arch springs from nearly the whole 

 length of their upper surface, and they have no anchylosed transverse processes. 

 In the Crocodile, where these processes are present, they are situated higher up on 

 the body of the vertebrae. In those vertebrae of the Cetacea, with which I have 

 compared the vertebra of the Zeuglodon, the transverse processes have the same 

 relative position, but they are more depressed. The Zeuglodon, therefore, in this 

 respect again shows a tendency to the higher mammiferous character ; the trans- 

 verse processes of the caudal vertebrae of the Manatee and Dugong are relatively 

 thicker than in the carnivorous Cetacea. Lastly, I may observe that the articular 

 surfaces of the body of this vertebra of the Zeuglodon, besides both being more 

 uniformly convex than in any saurian vertebrae which I have seen, also present 

 the strongly- marked concentric striae which characterize the flat or slightly con- 

 vex articular surfaces of the Mammalia. 



With respect to the other bones of the Zeuglodon, I may observe that the 

 fragments of the ribs present a thick, expanded form and dense texture, which 

 strikingly resemble the ribs of the Manatee, and clearly bespeak an aquatic 

 Mammal ; but the excentric laminated structure of these bones, which Dr. 

 Harlan has well described and figured, is peculiar, and unlike any structure 

 which I have yet observed in the ribs of either Mammal or Saurian, and it re- 

 sembles most the texture of the petro-tympanic bone of the Cetacea (PI. VIII. 

 fig. 5). 



One of the fragments of the ribs is decisive also against the supposed affinity 

 of the Zeuglodon to the Plesiosaur, since it presents an articular surface on the 

 tubercle as well as on the head of the rib. 



The hollow structure of the lower jaw has been adduced as evidence of the 



