Geological Society/. SOS 



test of the microscopic investigation of their intimate structure 

 with reference to the true affinities of the animal to which they be- 

 longed. The recent arrival of Dr. Harlan in England Avith the fossils, 

 and the permission which he has liberally granted Mr. Owen of 

 having the necessary sections made, have enabled him to determine 

 the mammlferous nature of the fossil. 



Among the parts of the Basllosaurus brought to England by Dr. 

 Harlan, are two portions of bone belonging to the upper jaw ; the 

 larger of them contains three teeth ; the other, the sockets of twoteeth. 

 In the larger specimen, the crowns of the teeth aremore or less perfect, 

 and they are compressed and conical, but with an obtuse apex. The 

 longitudinal diameter of the middle, and most perfect one, is three 

 inches, the transverse diameter one Inch two lines, and the height 

 above the alveolar process two inches and a half. The crown is trans- 

 versely contracted In the middle, giving its horizontal section an 

 hour-glass form ; and the opposite wide longitudinal grooves which 

 produce this shape, becoming deeper as the crown approaches the 

 socket, at length meet and divide the root of the tooth into two se- 

 parate fangs. The two teeth in the fore part of the jaw are smaller 

 than the hinder tooth, and the anterior one appears to be of a sim- 

 pler structure. 



A worn-down tooth contained in another portion of jaw, Mr. Owen 

 had sliced, and It presented the same hour-glass form, the crown 

 being divided Into two Irregular, rounded lobes joined by a narrow 

 isthmus or neck. The anterior lobe is placed obliquely, but the 

 posterior parallel with the axis of the jaw. The Isthmus increases 

 in length as the tooth descends In the socket until the isthmus finally 

 disappears, and the two portions of the tooth take on the character 

 of separate fangs. It Is evident that the pulp was originally simple, 

 but that it soon divided Into two parts, from which the growth of 

 the ivory of the teeth proceeded as from two distuict centres, now 

 separately surrounded by concentric striae of growth, the exterior 

 sending an acute-angled process into the isthmus. The cavitas 

 pulpi, which Is very small In the crown of the tooth, contracts as the 

 crown descends, and Is almost obliterated near the extremity, proving 

 that the teeth were developed fi-om a temporary pulp. 



The sockets In the anterior fragment of the upper jaw are indistinct 

 and filled with hard calcareous matter, but a transverse horizontal 

 section of the alveolar margin proves, that these sockets are single, 

 and that the teeth lodged therein had single fangs. In the anterior 

 socket, there Is an Indication of the transverse median contraction, 

 showing that this tooth resembled In form, to a certain degree, the 

 posterior tooth. A plaster cast of a portion of the lower jaw af- 

 forded the only means of studying this part of the fossil. It con- 

 tains four teeth, of which the two posterior are nearly contiguous, 

 the next is at an interval of an Inch and a lialf, and the most an. 

 terior of two inches from the preceding. The last tooth Is more sim- 

 ple in form tiian those 1)ehind, and it has been described as a canine. 

 'ITiis fragment of the lower jaw thus confirms the evidence afforded 

 by the fragments of the ui)pcr jaw, that the teeth in the Basilosaurus 



