34 WILLIAM K. GREGORY 
Great is the need for decisive evidence on this question, but 
before accepting Gaupp’s suggestion (11, p. 659) that the mam- 
malian and reptilian membranes were differentiated altogether 
independently, we may put forth the following purely provisional 
hypothesis embodied in figure- 23: that in the most primitive 
Cynodonts, such as Bauria, there was an extracolumella, resting 
against a tympanic membrane behind the squamosal, which had 
been differentiated out of the tissue lying between the endo- 
dermal epithelium of the tympanic cavity and the epidermis; 
Fig. 23 Hypothetical scheme showing the reptilian extracolumella and the 
mammalian manubrium mallei (= ? proce. retro articularis) both functioning at 
the same time. 
that, with the spread of the tympanic cavity (see p. 25) the 
differentiation of the future tympanic membrane also spread, 
until it included the stretched skin on the posterior end of the 
jaw, below the quadrate and articular and above the angular; 
that concomitantly with the reduction of the quadrate and articu- 
lar (p. 25) and the detachment of the angular and goniale from 
the dentary (p. 20), the newly differentiated portion of the tym- 
panic membrane became functionally more active than the old 
‘reptilian’ portion; that, in this way the old membrane together 
with the extracolumella became vestigial, while the new mem- 
brane became altogether free from the dentary, but remained 
