234- 



The adjustment of the cross wires in the exact sidereal focus of its 

 object-glass is, however, a point of the highest importance. 



The author next points out an important advantage which this in- 

 strument presents, viz. that of enabling the observer, by varying the 

 inclination of his float, to detect erroneous divisions of his circle by 

 bringing different parts of its arc into use ; after which he proceeds 

 to describe an application of his floating collimator, as a permanent 

 verification of the verticality of a zenith tube, and considers that by 

 its use the error, if any, in the zenith distance of a star, will be ulti- 

 mately referred to inaccurate bisection of the star, or imperfections 

 in the micrometer screws. 



Notice on the Iguanodon, a newly discovered Fossil Reptile, from the 

 Sandstone of Tilgate Forest, in Sussex. By Gideon ManteU, F.L.S. 

 and M.G.S. Fellow of the College of Surgeons, #c. In a Letter to 

 Davies Gilbert, Esq. M.P. V.P.R.S. #c. #e. $c. Communicated 

 by D. Gilbert, Esq. Read February 10, 1825. \PUl. Trans. 1825, 

 p. 179.] 



The bones of the fossil herbivorous reptile described in this paper 

 were discovered in the sandstone of Tilgate Forest in Sussex, which 

 is a portion of the iron-sand formation, and forms a chain of hilla 

 stretching in a W.N.W. direction from Hastings to Horsham. In 

 this sandstone the bones and teeth in question are accompanied with 

 those of saurian animals, turtles, birds, fishes, shells, and vegetables, 

 among which may be satisfactorily traced the remains of a gigantic 

 species of Crocodile, of the Megalosaurus, and of the Plesiosaurus. 



The teeth of the three last-mentioned animals are readily recog- 

 nised and identified; but in the summer of 1822, others were disco- 

 vered in the same strata, which, though evidently referrible to some 

 herbivorous reptile, possessed peculiar and striking characters. 

 Anxious to ascertain the opinions of naturalists respecting these, the 

 author submitted them to the inspection of the most eminent, and 

 among the rest to Baron Cuvier, who, while acknowledging that such 

 teeth were previously unknown to him, agreed in the conclusion of 

 their belonging to some herbivorous reptile of gigantic size, and re- 

 commended every research to be made for more connected portions 

 of the skeleton. 



Confirmed in his opinion by these remarks, the author renewed his 

 researches with increased assiduity ; and though no connected por- 

 tions of the skeleton have hitherto rewarded his pains, some of the 

 specimens were discovered in so perfect a state as to allow of a com- 

 parison with the teeth of recent lacertse in the Museum of the Royal 

 College of Surgeons ; and the result of this comparison was, that in 

 an Iguana there deposited, teeth were discovered possessing the form 

 and structure of the fossil specimens. 



Drawings both of the recent and fossil teeth accompany this paper, 

 and were exhibited to the Society. They show a striking corre- 



