PYTHONS AND BOAS. 
187 
the palate, the second and third likewise agree with the latter in having the 
shields on the lower surface of the tail mostly or entirely single; the tail itself 
being but slightly, if at all prehensile. 
The tree-boas of tropical America may be taken as examples of 
Tr66'Bod«s ** 
the second subfamily ( Boinon ) of the assemblage of snakes under 
consideration. The members of this subfamily can be distinguished from the 
preceding group solely by the absence of a supraorbital bone on the upper surface 
of the skull above the socket of each eye. They further differ from all the 
pythons, with the exception of two of the three genera last mentioned, in having 
STREAKED TREE-BOA (i nat. size). 
teeth on the palate; and, with the exception of the whole three of the connecting 
genera, in the absence of teeth in the anterior upper jawbones, or premaxillse. 
Moreover, the boas and their allies further differ from the typical pythons in 
having the shields on the lower surface of the tail for the most part single, 
thereby agreeing with the genera Aspidites and Calabaria ; and thus showing 
that the small group to which the two latter belong forms such a close connection 
between the pythons and boas as to preclude their reference to separate families. 
In common with the majority of the thirteen genera, into which the 
subfamily is divided, the tree-boas are characterised by having the head distinctly 
defined from the neck, and the tail more or less prehensile. They are specially 
distinguished by the anterior teeth being much larger than the hinder ones; by 
