MAMMAL-LIKE REPTILES. 785 
the manubrium of the malleus. With the reduction of the 
‘reptilian’ tympanic membrane the hyoid became separated from 
the extra-columella (as it does in many lizards) and migrated to 
a new insertion on the periotic.” 
The foregoing discussion will have shown how much of truth 
there may be in this brilliant hypothesis of Gregory’s. The 
suggestion that the stapes of Therapsids although directly arti- 
culated with the quadrate may have been connected with the 
membrane by an extra-columella, is exactly paralleled by the 
actual conditions in Mosasaurs, where the slender stapes is in- 
serted into a deep pit on the ner side of the quadrate, but is 
connected with the tympanic membrane by a process passing 
through the large special notch on the back of that bone. This 
process like the membrane is sometimes strongly calcified. The 
whole arrangement is after all only a further development of 
conditions commonly found in lizards. 
The facts as we know them in Diademodon, or rather the in- 
terpretations of those facts offered above, show that the reptilian 
portion of the tympanic membrane had actually been reduced to 
very small dimensions by a steady process connected with and 
no doubt induced by the same factor as the degeneration of the 
quadrate and the thinning of the basisphenoid and basioccipital. 
The facts also seem to show that the manubrinm mallei is 
altogether a new formation, developed whilst the articular was 
losing its suspensory function. 
If the explanation of the meaning of the observed series of 
changes in the articular region of the skull of Therapsids which 
has been presented above be true, or even if it contains but a 
small element of truth, it presents us with a good illustration of 
the fact, to my mind patent in the development of every organ 
of the body which is known, that the Therapsids from the moment 
of their initiation were committed to the final development of 
a mammalian structure. Different branches of them proceeded 
with their modifications to different degrees, and at very different 
speeds, but so far as the evidence goes, and it is no more than 
suggestive at best, their evolutionary change, in fundamental 
features, were always directed towards the final development of a 
mammalhan structure. 
I wish to acknowledge my indebtedness to the Percy Sladen 
Trustees, who assisted me in visiting the South African Museums. 
To Dr. Smith Woodward IL owe the opportunity of describing the 
material in the British Museum (Natural History), which is the 
basis of this paper, and I also owe to him and to Dr. C. W. 
Andrews thanks for their many kindnesses during my work at 
the Museum. Finally, I have to thank Mr. R. Hall, chief 
‘“mason” at the Museum, who developed the skulls described in 
this paper from a matrix which in many places is as hard as 
flint. 
