Mr. Owen on the Zeuglodon Cetoides. 73 



They differ from those of the great Mosasaurus and all the Lacertian Reptiles, 

 in being freely implanted in sockets, and not anchylosed to the substance of the 

 jaws. 



They differ from the teeth of the Ichthyosaurus in being implanted in distinct 

 sockets, and not fixed in a common and continuous groove. 



There remain then only the Plesiosaurus, Megalosaurus, Thecodonsaurus , and 

 the Crocodilian reptiles with which to compare the teeth of the Zeuglodon. Now 

 in these higher reptiles, which have the teeth implanted in distinct sockets, the 

 part of the tooth so inserted is always single, and maintains, as it descends, a broad 

 base, which is hollow. But in the Zeuglodon, on the contrary, the two fangs 

 diminish in size as they sink in the socket, and become consolidated by the pro- 

 gressive calcification of a temporary pulp. In all Saurians, moreover, there are 

 one or more new or successional teeth in different stages of formation, situated 

 close to, or contained in the cavity of the base of the protruded tooth ; but no 

 trace whatever of this characteristic Saurian structure has yet been detected in the 

 jaws of the Zeuglodon. 



If, therefore, I had had to give an opinion on the nature and afiinities of the 

 Zeuglodon from the obvious external characters alone of its teeth, I should have 

 concluded from these characters that it was no Reptile, but a mammiferous ani- 

 mal, most probably belonging to the cetaceous order, and more nearly aUied to 

 the herbivorous than to the piscivorous sections of that order as it now stands in 

 the Cuvierian System. 



As, however, the so-called Basilosaur has been regarded by some able anatomists 

 as affording an exceptional example among the Reptilia of teeth having two fangs, 

 contrary, indeed, to all analogy, so in like manner they may prefer to consider the 

 facts above-cited, relating to the solidification of the fangs and the absence of 

 numerous successional teeth, as exceptional dental conditions in an extinct Saurian 

 form, rather than as evidence of the mammiferous nature of the fossil in question : 

 I have therefore proceeded to investigate the intimate structure of the dental sub- 

 stance in the Basilosaurus , with a view to ascertain whether the evidence thus 

 afforded would be contradictory to the previous inferences of its mammiferous 

 nature, or give cumulative proofs of their correctness. 



I may premise, that the teeth of the Sphyroena and allied fossil fishes, which are 

 implanted in sockets, are distinctly characterized by a continuation of medullary 

 or pulp canals, arranged in a beautifully reticulate manner, extending through 

 the entire substance of the tooth, and affording innumerable centres of radiation 

 to extremely fine calcigerous tubes. 



In the Enaliosaurs and Crocodiles, the pulp-cavity of the tooth, which is analo- 

 gous to the subdivided medullary canals in the sphyrenoid fishes, is simple and 



VOL. VI. SECOND SERIES. L 



