96 REPORT — 1839. 



posterior apertures of the nostrils are thus thrown far back, as 

 in the Crocodiles. 



The transverse bones, or external pterygoids, complete the 

 boundaries of an aperture between the pterygoid and maxillary 

 bones ; and by abutting against the posterior extremities of the 

 maxillary bones greatly increase their strength. 



It appears to me, from a close inspection of some of the most 

 complete specimens of this intricate part of the skeleton of 

 the Ichthyosaurus in different museums, that the posterior 

 nostrils are not perforated, as in the Crocodile, exclusively in 

 the pterygoids, but that they occur in the interspace between 

 the internal pterygoids and the basi-sphenoid, as in the Lacer- 

 tians. With respect to the sphenoid, however, there is a struc- 

 ture characteristic of the Crocodiles, or at least not present in 

 those Lacertian crania which I have had opportunities of exa- 

 mining, viz. an oblique perforation of the basi-sphenoid for the 

 passage of the common termination of the Eustachian tubes. 

 The contour of the basi- sphenoid is heptagonal : the posterior 

 margin is the broadest, and is articulated by a thick rough sur- 

 face, with a corresponding margin of the basi-occipital. The 

 oval petrous bones are articulated to the sides of the sphenoid ; 

 each of the anterior lateral angles are produced, but not to the 

 same relative extent as in the Lacertians, in which they extend 

 as buttresses to the internal pterygoids : a moderately long 

 median slender pointed process is continued forwards from the 

 middle of the anterior surface of the sphenoid. The superior 

 or cranial surface of the sphenoid is traversed by a transverse 

 ridge. 



With reference to the lower jaw, it would be superfluous to 

 offer any observations after the admirable and accurate exposi- 

 tion of its composite structure which has been given in the 

 works of Conybeare, Cuvier, and Buckland. I shall therefore 

 limit myself to a comparison of its leading features with the pe- 

 culiarities of the two great divisions of the existing Saurian s. 



The dentary piece resembles that of the Lacertians and 

 differs from that of the Crocodilians in being pierced externally 

 by a few large vascular foramina disposed in a regular series. 

 It differs also from the Crocodilian type in terminating poste- 

 riorly above instead of beneath the anterior extremity of the 

 sur-angular piece. In the degree of development of the coro- 

 noid or complementary element the lower jaw of the Ichthyo- 

 saurus holds an intermediate place between the Crocodilian and 

 Lacertian groups ; it is of greater extent than in the Crocodile, 

 especially posteriorly, but does not send upwards a well-defined 

 process^ as in the Lacertians. The process analogous to the 



