1S90.] HELODERMA SUSPECTUM. 221 



lastly, the external fork of a palatine articulates with the maxillary 

 and the transpalatine or os transversum. With the pterygoid it 

 completes the inner periphery of the palatine foramen ; it forms its 

 entire anterior boundary, as it does the posterior boundary of the 

 internal narial aperture. 



An OS transversum is an important element in the lateral chain of 

 bones at the base of the skull. It is deeply cleft behind in the 

 horizontal direction, and into this closely fits the external limb of 

 the corresponding pterygoid, which is wedged for the purpose. 

 And it is thus that the pterygoid is extended to the maxillary, as 

 through the palatine it is by its internal fork extended to the vomer 

 of the same side. Articulating, then, with a palatine, with the 

 maxillary and the pterygoid, and touching the jugal to its outer 

 side, the os transversum completes the outer periphery of the palatine 

 foramen ; and also affords an important contribution to the osseous 

 floor of the orbital cavity. 



The pterygoids are a somewhat long and slender pair of bones. 

 Either one presents an enlarged anterior moiety and a straight and 

 slighter hinder shaft. These two portions form a curve which 

 presents its concavity to the outer side, and pressing against its 

 inner side at the middle is the extremity of the corresponding basi- 

 pterygoidal process. This latter is stout and prominent, and has 

 the appearance of pushing the pterygoid firmly against the quadrate 

 of the same side, as the columella appears to prevent it from rising 

 upwards. The pterygoid develops a small lip of bone at its ventral 

 side, which, extending backwards, overlaps the basipterygoid 

 process and thus prevents the slipping. As to its articulations, we 

 are to note tliat a pterygoid meets the columella, the basipterygoid 

 process the quadrate, while anteriorly it is powerfully wedged into 

 the OS transversum, suturally linked to the palatine, and barely 

 touches the jugal. To some extent a pterygoid assists to complete 

 the osseous flooring of the orbit, and it also completes the boundary 

 of the palatine foramen behind. 



Dr. Mivart has said in his article " Reptiles, ' in the !)th edition 

 of the ' Encyclopsedia Britannica (p. 451), that "The skull of 

 Heloderma is very remarkable in that it has no zygomatic arch 

 whatever." 



And this is commonly the way in which this fact is stated. It 

 is not, however, strictly true, for upon examining skulls of both old 

 and young individuals of Heloderma suspectuin I find at least a very 

 substantial rudiment of the arch in question. It also has been 

 noticed by Bocourt and by Troschel. It consists of a freely 

 articulated conical ossicle standing on top of the quadrate, being 

 moulded to the outer side of the posterior end of the squamosal, 

 with which it also freely articulates. It is seen to be present upon 

 both sides. That it is the osseous rudiment of the hinder end of 

 the zygomatic arch in this reptile there cannot be the shadow of a 

 doubt. 



The squamosals are well developed and occupy their most usual 

 position as seen in Lizards, being, upon either side, accurately moulded 



Proc. Zool. Soc— 1 890, No. XVI. 1 6 



