98 REPORT~1839. 



but they are not lodged in distinct sockets formed by the de- 

 velopment of bony partitions in this groove. 



The base of the teeth of the Ichthyosaurus is, however, 

 covered with a layer of cementum or true bone, which makes 

 the anchylosis of such pai't to the contiguous jaw quite possible ; 

 as it is through the medium of a like investment that anchylosis 

 actually does take place in the existing Lacertians. The pulp 

 of the tooth of the Ichthyosaurus, after it has been subservient 

 to the development of the crown of the tooth, becomes solidi- 

 fied in the fang or base by a coarse ossification which closes the 

 pulp-cavity at the lower part. Mr. Conybeare, who first pointed 

 out this fact, at the same time indicated the difference thereby 

 illustrated between the Tchthyosaur and Crocodile. But the 

 non-existence of anchylosis of the teeth to the jaw, and the de- 

 velopment of the inner wall of the alveolar groove, together with 

 the slight ridges intervening between the teeth, all tend, Mr. 

 Conybeare observes, to place the Ichthyosaurus much nearer 

 the Crocodilian than the Lacertian division of the existing Sau- 

 rians. 



Hyoidean Arch. — In three examples of the Ichthyosaurus, 

 Cuvier* detected the two horns or stylo-hyoid elements of the 

 hyoidean arch in their natural situation ; and he also states, 

 that he had seen between, and in advance of these lateral ele- 

 ments, an osseous disk, broader than long, notched posteriorly, 

 and which he suspected to be the body of the os hyoides. 

 These — the only elements of the hyoidean system which are 

 present in the skeleton of the Ichthyosaurus — are beautifully 

 displayed in their natural relative position in the Ichthyosaurus 

 lonchiodon in Mr. Hawkins's second collection, now transferred 

 to the British Museum. 



The cornua are robust, elongated, sub -prismatic bones, 

 slightly enlarged and truncate at both extremities : their junc- 

 tion with the small flattened hyoid body seems to have been 

 by means of abundant flexible ligamentous material : the length 

 of each cornu is a fifth part the length of the lower jaw. 



The condition of the hyoid apparatus is of great weight in 

 reference to the habits and affinities of the extinct animals in 

 question ; for in fishes, and the water-breathing reptiles, this 

 apparatus presents a magnitude and complexity proportionate 

 to its importance as the foundation of the branchial system. 



In the Lacertian Sauria, the os hyoides, though less compli- 

 cate than in Fishes, presents characteristic modifications in the 

 number and length of the lateral appendages which relate to 



* Ossem. Fossiles, torn. v. part xi., De I'lchthyosaurus, Art. iv., De I'Os 

 Hyoide. 



