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Show 1890.] 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, a? 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 ; aud 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, aud 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 that 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. I)r. Mivart has said in his article " Reptiles, ' in the 9th edition of the 'Encyclopaedia 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 suspectum 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 aud occupy their most usual position as seen in Lizards, being, upon either side, accurately moulded PROC. ZOOL. Soc-1890, No. XVI. 16 |