Henry L. Bruner 103 
cestors gave up the acquatic habits and the glandular skin of the amphi- 
bians and began to develop a thick protective epidermis which is char- 
acteristic of typical reptiles. As a result of these changes exuviation 
became difficult, especially in the head region, with its rigid skull and 
numerous openings. To overcome the difficulty the moulting mechan- 
ism was developed, probably by the use of a branchial muscle which had 
become more or less superfluous under the new conditions of life. This 
muscle was endowed with a new function and became the m. constrictor 
ven jugularis interne. 
Since the possessors of this moulting mechanism were ancestral to sev- 
eral orders of reptiles which have become extinct, it is not improbable 
that the mechanism existed in the latter, as well as in modern forms. 
According to Zittel, 87-90, the skin of these extinct reptiles, even that 
of the Ichthyopterygia and Sauropterygia, was relatively thick, and often 
provided with a horny epidermis. There is, therefore, reason to believe 
that the moulting mechanism was a necessity in these groups, as well as 
in others. 
In the pro-reptilia the moulting mechanism probably included a single 
special muscle: the m. constrictor venze jugularis interne. But with 
the differentiation of separate orders came a demand for a more effective 
mechanism, especially in certain groups. In the Testudinata greater 
efficiency of the mechanism was secured, apparently, by a stronger develop- 
ment of the constrictor muscle itself. The improved mechanism has 
met the needs of this group of sluggish animals. 
In the modern Ophidia the moulting mechanism shows a very simple 
condition, but it is doubtful if this condition is a primitive one, for since 
the Ophidia are descended from lizard-like forms, it is probable that the 
moulting mechanism of the snake has been modified in correlation with 
cther changes which have rendered the exuviation less difficult. In the 
Colubroidea and Solenoglypha these changes include the union of the 
eyelids and the development of great mobility of certain bones of the 
head. 
The highest form of the moulting mechanism has been developed 
among the Sauria, in which the m. constrictor venz jugularis is assisted 
by one or two other muscles which are especially concerned in the exuvia- 
tion of the anterior head region. The most primitive conditions, appar- 
ently, in this order are to be found in those forms which retain the second 
epibranchial cartilage. This cartilage gives attachment to the median 
part of the m. constrictor vene jugularis interne, which thus, in addi- 
tion to its newly-acquired function as a constrictor, still exercises the 
