Geological Society. 213 



of these tubes. Now the crowns of the teeth of the Basilosau- 

 rus evidently exhibit in many parts a thin investing layer of a 

 substance distinct from the body or ivory of the tooth, and the mi- 

 croscopic examination of a thin layer of this substance proves it to 

 possess the same characters as the csementum of the crown of the 

 tooth of the Dugong. The entire substance of the ivory of the teeth 

 consists of fine calcigerous tubes radiating from the centres of the 

 two lobes, without any intermixture of coarser medullary tubes 

 which characterize the teeth of the Iguanodon ; or the slightest trace 

 of the reticulated canals, which distinguish the texture of the teeth 

 of the Sphyraena and its congeners. The calcigerous tubes undulate 

 regularly, and also communicate with numerous minute cells ar- 

 ranged in concentric lines. 



Thus, the microscopic characters of the texture of the teeth of the 

 great Basilosaurus are strictly of a mammiferous nature, and con- 

 firm the inference respecting the position of the fossil in the natural 

 system drawn from the external aspect of the teeth. 



Mr. Owen then adduced further proofs of the mammiferous and 

 cetaceous character of the Basilosaurus, from the structure of the 

 vertebra?, from the great capacity of the canal for the spinal chord, 

 and from the form and position of the transverse processes, which 

 however present a greater vertical thickness than in the true Cetacea, 

 and approach in this respect to the vertebra? of the Dugong. 



With respect to the other bones of the Basilosaurus, Mr. Owen 

 stated, that the ribs in their excentric laminated structure are pecu- 

 liar, and unlike those of any Mammal or Saurian. The hollow 

 structure of the lower jaw of the Basilosaurus, which has been ad- 

 vanced as a proof of its saurian nature, Mr. Owen showed occurs 

 also in the lower jaw of the Cachalot, and is therefore equally good 

 for the cetaceous character of the fossil. 



In the compressed shaft of the humerus, and its proportion to the 

 vertebra?, the Basilosaurus again approximates to the true Cetacea, 

 as much as it recedes from the Enaliosaurians ; but in the expansion 

 of the distal extremity and the form of the articular surface, this hu- 

 merus stands alone ; and no one can contemplate the comparative 

 feebleness of this, the principal bone of the anterior extremity, with- 

 out agreeing with Dr. Harlan, that the tail must have been the main 

 organ of locomotion. 



Mr. Owen proposes to substitute for the name of Basilosaurus that 

 of Zeuglodon, suggested by the form of the posterior molars, which 

 resemble two teeth tied or yoked together. 



