406 ICHTHYOSAURUS IX THE MESOZOIC ROCKS OF X.-E. AUSTRALIA, 



dentary bones, and a portion of the left. At the anterior end the 

 specimen has been broken-off" short, immediately posterior to the 

 union of the rami, and at the hinder end just anterior to the 

 outer termination of the nasal bones. At the former the snout is 

 3J to 4 inches in transverse measurement, and at the latter point 

 4J, inches. There are thirty teeth in all preserved, ten implanted 

 in the pre-maxillary, and eleven in the dentary. 



Both bones are longitudinally channelled by a deep semi- 

 interrupted groove, similar to that seen in other species of 

 Ichthyosaurus. 



The teeth are of medium size, but larger than in some species, 

 measuring from 1 to IJ inches from the alveolar edge to the apex 

 of the crown, but including the implanted base, one of the fore- 

 most teeth measures 1| inches in length. They increase in 

 diameter and stoutness from before backwards, are conical sub 

 circular in section, with a non-trenchant, and apparently straight 

 crown. The enamel is ornamented with grooves and ridges, but 

 there is no evidence that the crown apices were devoid of these; 

 but on the contrary, the upper portions of the crown and the base 

 of the teeth above the alveolus, appear to be more strongly ridged 

 than does the middle line of each tooth. In the most anterior 

 tooth but one preserved in the pre-maxillary having a sectional 

 diameter of half an inch, there is a pulp cavity of three-sixteenths 

 of an inch. 



In a transverse section of the postei'ior end the following facts 

 are discernible. The breadth of the pre-maxillary across the top 

 of the alveolar cavity is 1 inch. On the outer border the 

 external groove traversing this bone longitudinally leads into 

 a well-marked cavity, the exterior alveolar wall being thin, but 

 this is perhaps owing to some extent to the lateral abrasion the 

 specimen has undergone. The alveolus cut by the section is five- 

 eighths of an inch wide, and 1 inch in depth. From the thickened 

 and enlarged lower end the inner wall of the bone is directed 

 upwards in a fairly straight line until it reaches a point on a level 

 with the upper end of the alveolus. Here it takes a well-marked 



