30G Geological Society. 



Sphyrtena and its congeners. The calcigerous tubes undulate regu- 

 larly, and, like those of the Dugong, exhibit more plainly the pri- 

 mary dichotomous bifurcations, and the subordinate lateral branches 

 given off at acute angles : they also communicate with numerous 

 minute cells arranged in concentric lines. 



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

 great Basilosaurus are strictly of a mammiferous nature ; and Mr. 

 Owen further showed that they differ from those of the fossil Eden- 

 tata, which are also surrounded by csementum, in the abseace of the 

 coarse central ivoi-y ; and confirm 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 

 vertebree, which proves that the epiphyseal laminaj were originally 

 separated from the body of the vertebrae, but were afterwards united 

 to it. In the bodies of the smaller vertebrae the epiphyses are 

 wanting, and Mr. Owen agrees with Dr. Harlan in inferring from the 

 common occurrence of this condition, that there were originally 

 three separate points of ossification in the body of the vertebrae ; a 

 character never noticed in the vertebrae of Saurians, but a most pro- 

 minent one in those of the Cetacea. Another argument in favour 

 of the mammiferous and cetaceous nature of the Basilosaurus is de- 

 duced 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 neura- 

 pophyses as compared with that of the body of the vertebrae ; in 

 their regular concave posterior margin, and the development of the 

 articular apophyses only from their anterior part : also in the form 

 and position of the tran.sverse processes, which however present a 

 greater vertical thickness than in the true Cetacea, and approach in 

 this respect to the vertebrae 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 

 vertebrae, 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, in compliance with the suggestion of Dr. Harlan, who, 

 having compared with Mr. Owen the microscopic structure of the 



