412 Mr. G. A. Boulenger on the Distinctive 



XLIX, — On the Distinctive Cranial Characters of the Iguano id 

 Lizards allied to Iguana. By G. A. Boulengek. 



Shoetly after the publication of the second volume of the 

 British Museum ' Catalogue of Lizards ' Prof. Cope proposed 

 an arrangement of the genera of Iguanina, {. e. of the genera 

 closely allied to Iguatia^ " without abdominal ribs or free 

 dermal margins of the digits, with the nostrils on the line of 

 the canthus rostralis and not below it, and which possess the 

 compressed form and other characteristics indicating an arbo- 

 real rather than a terrestrial habit of life " *. This arrange- 

 ment is certainly no advance on that which I had previously 

 followed, the only important innovation being the union of 

 the genera Metopoceros and Gyclura under the latter name. 

 His reasons for doing so ai'e given in the following words : — 

 " If the presence of the second row of femoral pores is not 

 constant in G. cornuta, then the genus Metopoceros cannot be 

 distinguished from Cyclura. Mr. Boulenger relies on the 

 rather greater number of denticles in the lateral teeth in G. 

 cornuta^ but my specimens show a tendency to the tridentate 

 form of C. nuhila. The character is, I think, even if constant, 

 insufficient for generic distinction." Although agreeing now 

 with Prof. Cope as to the value of the latter character, to 

 which I attached too much importance, I yet wish to uphold 

 the distinction of the genera Gyclura and Metopoceros on the 

 ground of the cranial structure. Although closely allied to 

 Gyclura^ Metopoceros is, in some respects, equally related 

 to Iguana^ whilst the skull of Gyclura stands nearer to that 

 of Gtenosaura than to that of Metopoceros. 



On this occasion I propose to indicate the distinctive cranial 

 and dental characters of the genera more nearly related to 

 Iguana. 



1. Amhlyrhynchus, Bell. — All the teeth trilobate. Prte- 

 maxillary not extending as far as the posterior border of the 

 nasal fossae ; the length of the latter nearly equals their dis- 

 tance from the orbits. Prefrontal not entering the nasal 

 fossa. Postfronto-squamosal arch short, not longer than the 

 orbit ; postfrontal as long as deep. Transpalatine in contact 

 with palatine. Basispiienoid short and much constricted 

 behind the basipterygoid processes. 



* rroc. Amer. Phil. Soc. xxiii. 1S80, p. 2G1. 



