AND CLASSIFICATION OF THE FOSSIL llEPTILIA. 
201 
seems to me probable that this development is the cause for the scattered positions 
of the otic bones, and that when the scjuamosal becomes smaller those bones come 
into closer relation. 
The skull structure is especially suggestive in relation to elements in the Mammalian 
auditory region which are not found in Reptiles. In Echidna two elements are seen, 
one an imperfect circle, and another external and anterior to it. The former of these 
is in contact with the pterygoid, just as is the quadrate bone in Reptiles and Birds. 
The latter is extended between the squamosal and the pterygoid, and meets the 
tympanic ring. It is regarded as the malleus. This bone almost exactly corresponds 
m position with the bone in the Dicynodont skull which has been often referred to in 
my descriptions of the palate. The tympanic ring similarly corresponds to the 
quadrate bone ; and the relations of malleus and tympanic in Echidna to each other, 
and to the surrounding skull bones, are almost exactly those of the quadrate and malleus 
in Anomodonts, though both bones are relatively much larger in the Reptile than in 
Mammals. Hence it seems to follow that when the squamosal came to extend outside 
the quadrate and in front of it, taking on itself part of the function of forming the 
articulation for the lower jaw, that the quadrate and malleus would be thrown inward 
and backward, and diminish in size at the same time. Some steps in this process of 
degeneration are seen in Anomodonts, and they are all approximations towards the 
Mammalian type. 
The difficulty in harmonizing the composite structure of the Reptilian lower jaw 
with the simple Mammalian jaw is similar to the difficulty with the composite roof bones 
of the Reptilian skull. In the most Mammal-like of Reptiles, Gcdescmriis, the lower 
jaw remains as Reptilian as in a Chelonian or Crocodile. The Mammalian might be 
derived from the Reptilian mandible, in one of two ways. It may be supposed that 
the elements forming the lower jaw ceased to be segmented, as we have assumed in 
explanation of the roof bones of the skull, and, therefore, tliat the Mammalian jaw 
includes the same elements as the Reptilian jaw, but in an undifferentiated condition. 
In favour of this view it might be urged that a yet more improbable development of a 
like kind is seen in existing Chelonians, where the dentary elements of the opposite 
sides lose their individuality, and form a single dentary element which unites the rami. 
But, perhaps, it may be as well to rejnember. before following this speculation further, 
that the articular element of the lower jaw would necessarily undergo a certain change 
of function akin to those of the quadrate bone, by which it shares the articulation with 
the sur-angular element in the same way as the quadrate shares the articulation with 
the squamosal. And, if the articular hone ceases to make the joint with the quadrate, 
owing to the abstraction of the quadrate from such work in the skull, it should result 
that the articular bone ceases to be ossified, because the mechanical conditions which 
determined its ossification have disappeared. The lower jaw is distinctly formed 
about Meckel’s cartilage ; and, whereas the articular bone is an ossification at the 
terminal end of that cartilage, and the only part of it which is ossified, it is instructive 
2 P 2 
