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558 

is strikingly similar to that of Chrysemys. From the present limited 
knowledge of the reptilian tympanum, it seems likely that the simpler 
one, formed from the hyo-mandibular pouch alone, is the more pri- 
mitive type, and that Testudo graeca may present an aberrant form. 
Between the quadrate and the otie capsule the blood vessels 
and nerve are again found. The course of the A. carotis interna has 
become more ventro-dorsal, and passing below the shaft of the colu- 
mella, it has Joined by its branch the A. facialis. In the younger em- 
bryos the chorda tympani passed above the columella in the region 
of the extra-columella, but in these later stages it crosses the median 
part of the shaft. It no longer extends along the outer lateral, but 
the median wall of the quadrate to the inner side of the mandible. 
Fourth Stage of Development. 
Two specimens of 12 and 15 mm. carapace measurement, have 
furnished the material for this description. But little remains to 
be said in this or the followmg stage, because the fundamental 
relations of the adult have already been etablished. 
The straight shaft of the columella of earlier stages has now a 
slight dorsal arch in the distal portion at the point where tympanic 
cavity has grown around it. Chondrification is complete and the 
boundary between the two parts of the columella remains well marked. 
As the condensations of the quadrate and extra-columella have 
been differentiating, there have gradually appeared around their 
outer lateral faces, fibrous strands of tissue. In this stage it has 
formed a continuous band which spans the quadrate and holds the 
extra-columella in place. This of course is the tympanic membrane, 
or at least its middle fibrous layer. From all appearances its origin 
is not associated with the closing of the gill clefts because it appears 
iate; it seems to arise, as it does in Testudo graeca, as a secondary devel- 
opment in connection with the quadrate and extra-columella conden- 
sations. . 
Young Adults (Fig. 9). 
The following remarks are based upon the specimens (carapace 
measurements, 19, 21, and 26 mm.). The quadrate has become firmly 
attached to the capsule and its caudal end is completely enveloped 
by the squamosal. The tympanum has entirely filled the hollow 
prepared for it. Although ossification has not actually started, indi- 
