52 G. S. WEST. 



nature as that found between all the scales of the head (cf. 

 PI. 4, fig. 14). 



The inner chamber (PI. 4, figs. 14 and 17, i. c.) lies 

 deeper than the outer one, and is rather more posterior in 

 position, being situated immediately in front of the orbit, 

 and occupying a hemispherical hollow on the outer face of 

 the maxillary bone. Its posterior wall abuts against the 

 anterior wall of the orbit, and in the largest animals exa- 

 mined these walls were very thin and semi-transparent. 

 The outer wall of the inner chamber also forms the inner 

 wall of the outer chamber, and consists of a membranous 

 partition which I shall subsequently refer to as the '^pit- 

 membrane." The cavity is not a completely closed one, but 

 communicates with the exterior by a minute pore at the 

 anterior angle of the orbit. As this pore lies under the 

 posterior margin of the lower antorbital scale it is not exter- 

 nally visible, but on reflecting the border of this scale it is 

 easily discernible (cf. PI. 4, fig. 6, o. i.), and readily admits of 

 the introduction of a bristle. 



The inner chamber was mentioned by Russell and Home ^ 

 as an oval cavity between the pit and the eye. Its interior is 

 lined throughout by a continuous thin cuticle of not more 

 than 1 ju in thickness. In some crotalines (e. g Lachesis 

 gramineus) the cuticle is crenulated on both the internal 

 and external walls of the chamber, but in others (e. g. 

 Crotalus triseriatus, 0. terrificus) only that of the 

 internal wall is crenulated, that of the external wall being 

 quite smooth. 



This crenulated cuticle is of precisely the same nature as 

 that lining the depressions between the scales on the exterior 

 of the animal, and consists of an expanded sheet of knob-like 

 elevations, each of which is distinct from its neighbours. 



> Russell and Home, 1. c. ; tiiey inferred from the situation of these oval 

 cavities that " they must be considered as reservoirs for a fluid which is 

 occasionally to be spread over the cornea; and they may be hlled by the 

 falling of the dew, or the moisture shaken from the grass through which the 

 snake passes." 



