118 REPORT— 1839. 



the intermaxillaries, and their analogues the dentary pieces of 

 the lower jaw. These latter pieces have a longitudinal groove 

 on their external surface near the alveolar ridge. The suran- 

 gular disappears beneath the dentary about half an inch anterior 

 to the nostril ; the angular continues longer visible on the out- 

 side of the jaw. 



The teeth are more slender in proportion to their length 

 than in any of the previously described species : I count from 

 65 to 70 on each side of the upper jaw ; of these the posterior 

 third, or about 25, are implanted in the slender maxillary bones. 

 In the lower jaw there are about 60 teeth on each side. They 

 are directed more obliquely backwards than in the species pre- 

 viously described. 



The parietals are divided by a persistent sagittal suture, and 

 the foramen is principally situated in this suture, the anterior 

 part only encroaching between the frontals. Each of the post- 

 erior parietal bifurcations runs parallel with, and is applied to 

 the outside of the supraoccipital bone. The median frontals 

 are also separated by suture ; they are relatively larger than in 

 communis, but do not reach the margin of the orbit. The 

 anterior frontals lie on the outside of the median frontals ; the 

 posterior frontals on the outside of the parietals. Cuvier states 

 that the post-frontals form the whole of the posterior boundary 

 of the orbit. In the Ich. tenuirostris this boundary is slender, 

 and presents a fine and deep smooth groove next the orbit. 



I have already alluded to the large size of the orbits : they 

 are not less characterized by the slenderness of their inferior and 

 posterior parietes ; the diameter of the orbit equals that of the 

 posterior or occipital region of the cranium. 



The malar bone is singularly long and slender, and brings to 

 mind its characteristic condition in Birds : its posterior extremity 

 is joined with a slender curved descending process of the zygo- 

 matic bone. The rest of the zygomatic element is much more 

 robust, and passes obliquely backwards to join the articular ex- 

 tremity of the tj'^mpanic bone, and to circumscribe the temporal 

 fossa below. The temporal fossa is bounded by the posterior 

 frontals, the parietal fork, and by a bone which Cuvier regards 

 as peculiarly Ichthyosaurian, and which extends from the post- 

 frontals to the end of the parietal fork. The nasal bones have 

 the margin which forms the upper boundary of the nostrils, 

 slightly convex, encroaching upon the nostril. The nostrils are 

 narrow and elongated, but apparently larger in proportion than 

 in the other Ichthyosauri, measuring two inches and a half 

 long in a head two feet in length now in the British Museum, 

 and figured by Mr. Hawkins in his 13th Plate. The rami of 

 the lower jaw soon unite, and the symphysis extends through 

 more than the anterior two-thirds of the jaw. 



