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U 
Henry L. Bruner 
is present and Platydactylus has also the m. protrusor oculi accessorius. 
In both species the venous sinuses are well developed in the orbital region 
and in the anterior part of the head. On the other hand, the m. con- 
strictor vene jugularis interne is entirely wanting and the postorbital 
veins are little, or not at all, enlarged. The operation of this modified 
mechanism must be more or less peculiar, but no observations have been 
made on the mechanism in action. 
C. ON THE SIGNIFICANCE OF THE SWELL MECHANISM OF 
THE SAURIA. 
a. A Mouttinec MecHANIsM. 
The first theory entertained by the writer in regard to the final sig- 
nificance of the swell mechanism of the head of the Sauria was suggested 
by the well-known habit of many lizards to inflate the body for protective 
purposes or for other reasons. Carus, 34, calls attention to the inflation 
of the mouth or special laryngeal sacks as a means of sexual attraction 
or for frightening enemies. Leydig, 72, says that Lacerta enlarges the 
body by strong inflation of the lungs, and a similar habit occurs also in 
Phrynosoma, in which it serves to erect the numerous dorsal spines. 
According to Dekay, 42, Sceloporus elevates its spines when irritated, so 
as to present a formidable appearance. Still more remarkable adaptations, 
presumably for the same purpose, are the “ frills” of Chlamydosaurus 
kingii, the gular sacks of Metopocerus cornutus, and many other Igua- 
nide, the dilatable occipital sack of Basiliscus, ete. 
In all of these cases it is reasonable to suppose that the effect might 
be augmented by enlargement of the head, and especially by the pro- 
trusion of the eyes. It has been suggested by Hay, 92, that Phrynosoma 
ejects blood from its orbital sinus as a means of frightening its enemies, 
and it is not improbable that in this case, the mechanism for elevating 
the blood-pressure is used as a fright mechanism. Such a function, 
however, is not sufficient to explain the wide distribution of the mechan- 
ism. On the contrary, it is probable that the flooding of the cephalic 
sinuses for frightening enemies is at best only a secondary use which 
has been acquired by relatively few forms. 
There is still less evidence that the swell mechanism is used for sexual 
purposes. The mechanism is equally developed in both sexes and there 
is no reason to believe that it is employed as a means of sexual attraction. 
A very important function of the swell mechanism of the Sauria has 
been discovered by a study of the moulting habits of these reptiles. Obser- 
vations pointing to such an explanation were first made by the author, 98, 
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